if it's still able to move, you might be able to point a power head at it, or move the rock it its on into a high flow area of the tank and see if it is capabe of moving itself out of the way, it would be a lot easier than trying to pry whats left of the poor critter off the rock and risking the possibility of killing the poor thing, if it isn't already dead.
If you have a qt tank set up, especially if it has the proper lighting, or lighting that would be suitable for an anenome to be nursed back to health, you could try moving it and the rock it is on in there.
Are the tentacles still sticky?? If so then there might be some hope. Is it bleached in color?