Ooops sorry...was absent for a very long time...

First of all i would like to know is there any other inhabitants in the tank ?
DO u have any places for them to hide? What kind of pleco is that ?

If its a common Pleco, then there shud be no problem as this guys are very very hardy...trust me...mine refuse to die...i left him in the bucket with water level lower than his body height for about 3 hours...and he is still menacing my cichlids...

Another thing is that if its stuck to the wall which is covered with algae, most probably he is lookin for something to chew.

Mine always hide...have about erm 20 plecs to date. What turns them on is a piece of cucumber or zuchinni...they will be scared but they will come out to eat regardless if u r in front of them. plecs are generally nocturnal which means they only come out to feed when its dark. But mine's got a few screw loose as they sleep in the night and feed during the day...