Personalize a rectangle node tikz

One possibility might be to add a path picture to the style, e.g.

path picture={
   \fill [green!50!black] ([xshift=-4mm]path picture bounding box.north east) -| ([yshift=-4mm]path picture bounding box.north east) -- cycle;
  }

So if you have

noeudA/.style={
  rectangle,
  inner sep=0pt,
  text width=25mm,
  align=center,
  draw=black,
  fill=green!30,
  minimum height = 10mm,
  path picture={
   \fill [green!50!black] ([xshift=-4mm]path picture bounding box.north east) -| ([yshift=-4mm]path picture bounding box.north east) -- cycle;
  }
  }

Then the first node of your diagram will look like

Alternatively you could make a new upperlefttriangle style where the color is an argument, i.e.

upperlefttriangle/.style={
path picture={
   \fill [#1] ([xshift=-4mm]path picture bounding box.north east) -| ([yshift=-4mm]path picture bounding box.north east) -- cycle;
  }
}

And then use

upperlefttriangle={<color>}

either in other styles, or in individual nodes.

MWE, much more concise as provided MWE (after a some, actually lot of effort :) ) combine trees and positioning library:

\documentclass[tikz, margin=3mm]{standalone}
\usetikzlibrary{chains, positioning, trees}
\usepackage[utf8]{inputenc}
\newcommand\ppbb{path picture bounding box}

\begin{document}
\begin{tikzpicture}[
    node distance = 2mm and 2mm,
    start chain = going below,
base/.style = {rectangle, draw,
               fill=#1,
               inner sep=0mm,
               minimum height = 9mm, text width=25mm, align=center,
               font=\linespread{.9}\selectfont
               },
   T/.style = {path picture={\fill[cyan!75]
                (\ppbb.east) -- (\ppbb.north east) -- ++ (-4.5mm,0) -- cycle;
                             }% end of path picture
               },
boxB/.style = {base=#1, text width=20mm},
% on chain
 box/.style = {boxB=#1, on chain},
% tree's parameters
  level distance = 18mm,
sibling distance = 32mm,
  level 2/.style = {sibling distance = 25mm},
  level 3/.style = {sibling distance = 22mm}
                        ]
% first part of image
% start of tree
\node[base=teal!30] {SCR}
    [edge from parent fork down]
    child { node[base=teal!30]      {Ajustement}}
    child { node[base=teal!30,T]    {BSCR}
        child { node[boxB=teal!15,T]    (Rba)   {Marché}}
        child { node[boxB=teal!15,T]    (Rbb)   {Santé}
            child { node[boxB=teal!10,T](Rca)   {SLT}}
            child { node[boxB=teal!10,T](Rcb)   {Catastrophe}}
            child { node[boxB=teal!10]  (Rcc)   {NSLT}}
                }
        child { node[boxB=teal!15,T]    (Rbc)  {Contre-partie}}
        child { node[boxB=teal!15,T]    (Rbd)  {Vie}}
        child { node[boxB=teal!15]      (Rbe)  {Non-vie}}
        child { node[boxB=teal!15]      (Rbf)  {Actifs incorporels}}
            }
    child { node[base=teal!30]  {Opérationnel}};
% Noeud Marché
{[start chain]
\coordinate[on chain,below=of Rba.west |- Rca.south]   (a);
\node[box=blue!10,T] (Rbaa) {Taux d'intérêt};
\node[box=blue!10,T] (Rbab) {Action};
\node[box=blue!10,T] (Rbac) {Immobilier};
\node[box=blue!10,T] (Rbad) {Spread};
\node[box=blue!10,T] (Rbae) {Change};
\node[box=blue!10,T] (Rbaf) {Concen\-tration};
\node[box=blue!10,T] (Rbag) {Illiquidité};
}
% Noeud Santé
{[start chain]
\coordinate[on chain,below=of Rcb.south west]   (b);
\node[box=blue!10,T] (Rbbaa)  {Mortalité};
\node[box=blue!10,T] (Rbbab)  {Longévité};
\node[box=blue!10,T] (Rbbac)  {Incapacité Invalidité};
\node[box=blue!10,T] (Rbbad)  {Rachat};
\node[box=blue!10,T] (Rbbae)  {Dépenses };
\node[box=blue!10,T] (Rbbaf)  {Révision};
}
% Noeud NSLT
{[start chain]
\coordinate[on chain,below=of Rcc.south east -| Rbc.east]   (c);
\node[box=blue!10] (Rbbca)  {Prime et réserve};
\node[box=blue!10] (Rbbcb)  {Rachat};
}
% Noeud Vie
{[start chain]
\coordinate[on chain,below left=of Rcc.south -| Rbe.west]    (d);
\node[box=blue!10,T] (Rbda) {Mortalité};
\node[box=blue!10,T] (Rbdb) {Longévité};
\node[box=blue!10,T] (Rbdc) {Incapacité Invalidité};
\node[box=blue!10,T] (Rbdd) {Rachat};
\node[box=blue!10,T] (Rbde) {Dépenses};
\node[box=blue!10,T] (Rbdf) {Révision};
\node[box=blue!10,T] (Rbdg) {Catas\-trophe};
}
% Noeud Non Vie
{[start chain]
\coordinate[on chain,below left=of Rcc.south -| Rbf.west]    (e);
\node[box=blue!10,T] (Rbea)   {Prime et réserve};
\node[box=blue!10,T] (Rbeb)   {Rachat};
\node[box=blue!10,T] (Rbec)   {Catas\-trophe};
}
% lines a
\coordinate[left=of Rbaa.west] (aa);
\draw   (Rba) -- ++ (0,-0.9) -| (aa) -- (Rbaa);
\foreach \i [remember=\i as \ii (initially a)] in {b,c,...,g}
    \draw (aa |- Rba\ii) |- (Rba\i);
% lines b
\draw   (Rca) |- (Rbbaa);
\foreach \i [remember=\i as \ii (initially a)] in {b,c,...,f}
    \draw (Rca |- Rbba\ii) |- (Rbba\i);
% lines d
\draw   (Rcc) |- (Rbbca);
\draw   (Rcc) |- (Rbbcb);
% lines e
\draw   (Rbd) |- (Rbda);
\foreach \i [remember=\i as \ii (initially a)] in {b,c,...,g}
    \draw (Rbd |- Rbd\ii) |- (Rbd\i);
% lines f
\draw   (Rbe) |- (Rbea);
\foreach \i [remember=\i as \ii (initially a)] in {b,c}
    \draw (Rbe |- Rbe\ii) |- (Rbe\i);
\end{tikzpicture}
\end{document}

Addendum: in the case, when all nodes in second part of block diagram have the same color, than the code of this part can be further reduced:

% second part of image
    \begin{scope}[every node/.append style = {box=blue!10}] % define common style for all nodes in scope   
% Noeud Marché
{[start chain]
\coordinate[on chain,below=of Rba.west |- Rca.south]   (a);
\node[T] (Rbaa) {Taux d'intérêt};
\node[T] (Rbab) {Action};
\node[T] (Rbac) {Immobilier};
\node[T] (Rbad) {Spread};
\node[T] (Rbae) {Change};
\node[T] (Rbaf) {Concen\-tration};
\node[T] (Rbag) {Illiquidité};
}
% Noeud Santé
{[start chain]
\coordinate[on chain,below=of Rcb.south west]   (b);
\node[T] (Rbbaa)  {Mortalité};
\node[T] (Rbbab)  {Longévité};
\node[T] (Rbbac)  {Incapacité Invalidité};
\node[T] (Rbbad)  {Rachat};
\node[T] (Rbbae)  {Dépenses };
\node[T] (Rbbaf)  {Révision};
}
% Noeud NSLT
{[start chain]
\coordinate[on chain,below=of Rcc.south east -| Rbc.east]   (c);
\node  (Rbbca)  {Prime et réserve};
\node  (Rbbcb)  {Rachat};
}
% Noeud Vie
{[start chain]
\coordinate[on chain,below left=of Rcc.south -| Rbe.west]    (d);
\node[T] (Rbda) {Mortalité};
\node[T] (Rbdb) {Longévité};
\node[T] (Rbdc) {Incapacité Invalidité};
\node[T] (Rbdd) {Rachat};
\node[T] (Rbde) {Dépenses};
\node[T] (Rbdf) {Révision};
\node[T] (Rbdg) {Catas\-trophe};
}
% Noeud Non Vie
{[start chain]
\coordinate[on chain,below left=of Rcc.south -| Rbf.west]    (e);
\node[T] (Rbea)   {Prime et réserve};
\node[T] (Rbeb)   {Rachat};
\node[T] (Rbec)   {Catas\-trophe};
}
    \end{scope}

The result is the same as before.