Android - Scan Barcode and QR code from the mobile screen itself

As already described in above comments, this would require taking a screenshot and saving it as an image file. Havind done that, there are several barcode scanners available in the playstore supporting "loading" of barcodes from image files, as e.g. QR BARCODE SCANNER or mentioned Google Goggles. I don't know of any barcode scanner capable of "scanning your screen directly".

EDIT: QR Droid seems to be capable of what you're looking for:

Scan QR codes from your camera, browser, SD card, or saved image