Yesterday I picked up one of the rocks in my tank and underneath it was a black spindly fuzzy creature with 8 legs or tentacles? It slunk/swam under another rock. It had a diameter of about 3 inches the (legs) were very skinny , fuzzy and I think there were 8 of them. Any clues about what this might be be? Don't know where it went now but it is lurking in there.