How can I stop seeing hidden files in the Midnight Commander?

or you can toggle it with Alt-. (Alt-period)

Choose Options from the menu bar, then Panel options. You have it right there, 5th option on the left column: "Show hidden files".

You might also try to toggle hidden files with Esc+. (escape + period), if Alt+. does NOT work.

This keyboard shortcut works on my Debian+xfce installation.

Note, that originally it is M-., where M stands for Meta key. You can find more (fascinating!) information about it in the @Gilles answer here: What does m-tab mean?

If you still prefer to use Alt+. instead of Esc+. you have to enable something like Meta Sends Escape in Your terminal.

In xterm, for example, You can do it by holding the Ctrl while clicking and holding the left mouse button. That pops up a menu where You can select Meta Sends Escape.