Hello! We are 3 weeks in with our salt water tank... we unfortunately got advice from the people at petco and have had a very rough start to the tank life! After unfortunately losing a couple of fish due to adding to many at once we have backed off and working on getting the water correct! I have these little guys and im curious on what they are. (Pictures of what i am talking about). Currently i have one watchman, one clown (his partner died), one chocolate chip starfish, and 5 crabs. The other 3 died (again wrong info from petco). I am currently fixing the issues i have caused and these tank mates i have now are strong and bearing with me. So my question is what do i have on my rocks and is it harmful??