how to cover places and erase the uncovered.

hi guys, i just want to know if there is a way to cover a picture with a layer that covers some areas of the picture and some not. So, i want to erase the uncovered side

of the picture not the layer.

EXAMPLE:

in this example i want to erase the grey blocks without erasing the covered ones by the brown lines. and also is there an anti erase tool? THANKS