They are algae grazers and don't hurt anything. Good luck trying to pry one off a rock though. I've had one in ,y tank for about 3 yrs now. First time I saw him, tried to pry him off the rock with a butter knife, but his suction power was so intense, broke a piece off his shell before he would let go. They're just like a large wood louse. Curls up in a ball for defense.