Could you get a closer pic? I'm not sure of the species in the pic. it sort of looks like a BTA, which wouldn't be happy on top of a rock like that and it also wouldn't be happy on the gravel. They like to attach inside a hole or cave in the rock. When wandering, they unfortunately are too often destroyed by either a powerhead intake or filter intake. You need to cover intakes with plastic window screen available at Home Depot/hardware stores.