A forgotten fish tank

Question
This fishtank is one of two, the care giver of the fish tank is gone for weeks and I found the whole thing covered in thick slimy moss (I'm guessing algae) Also, I believe I saw the tail of  a dead fish because while I was trying to clean it up the tail wouldn't move. I was about to take both the dead fish and the fake coral it was stuck in out when I thought I saw movement. For five or ten minutes I didn't know if it was really alive as I chipped away, sure enough it's breathing and has a healthy tail wag whenever I try to get it free. (Yes.. Poor fish being stuck with me.) My question is what is most likely the reason why the fish even with it's back and back fin fully exposed would stay inside there. It looks as if it's cramp and I can't see both fins or it's mouth.
[It's a bottom feeder about the length of 6+ inches and a healthy diameter of a few inches.

Answer
Hi James:  take a net and clean away all of the floating algae.  You will need to also drain about 75% of the water and replace it... You probably saw a plecostomus in there and I would just let him be until he is ready to come out.  It probably feels safe in there.  They are more of a night time creature...so that may be its day time hinging spot.  Just take out most of the water and refill it with tap water... if you see water condition there you can use that but for the moment the chlorine in the tap water will help to kill the algae.  Look around and see if you see anything around the tank with a long clear looking hose and a fatter plastic tube connected to it... that would be a gravel vacuum... let me know if you find one... dave