plecos do not eat slime coatings, they do sometimes attach themselves to fish with a wide flat body such as a discus, but not to feed off of it. A pleco ideally wants to eat algea off of a piece of wood (the cellulose in the wood aids their digestion). What kind of cray did you add? Its not a predatory species is it? I dont know what species of crays you get there but all our crays will eat fish. If it was an electrical short Id assume it would effect all the tank inhabitants not just one (aswell as you when you put your hand in there). If it keeps doing this it does have the potential to do damage to its face, consider giving the fish a place to hide, if you havnt allready done so, such as a piece of wood. This will give it a place to relax. Maybe its just getting a fright and is trying to escape, observe it from outside a window and see if it does it when you arent staring at it, then come into the room and see if it starts up again. You might have to repeat this more than once because it might not do what it normally does the first time for some reason. This doesnt seem likely but try anyway you never know.