Oh yeah, that's a micro brittle star! If you see one, you have at least a dozen. Perfectly harmless, rather beneficial. They feed on stuff too small to see. The arms are different lengths because they are, well, brittle. They break easily.
The only way I know of for them to cause any problems at all is if they annoy polyps such as zoas with their constant arm waving.
Population is limited only by available food and predation.