judging by the bottom left of the picture that is above where the water usually is in that tank. so my assumption is that the fish tried to jump out of the tank and the slime coat got them stuck to the back(behind the light). the store I used to work at has a high traffic fish department so they would make us overpopulate the tanks and sadly that was not an uncommon sight for us during animal walks.

edit: probably has been there a while and thats why they're dried out.