It is pretty much impossible to tell if I is harmless with that description. A good rule of thumb is that ALL crabs are bad. Yeah, some people add emeralds or whatever, but all crabs are omnivores and opportunity feeders. If you did not buy it as reef safe there is a really good chance it is not- and if you have coral or inverts then it may be best to get rid of it. Even if you did purchase a reef safe crab I would be careful. I can send you the picture of my emerald eating a hermit (they both came with the tank and are long gone...) As for eggs- look here
http://images.google.ca/images?hl=en...-8&sa=N&tab=wi and you will see that your description fits the look of crab eggs. Anyhow, possibly post a picture of the crab for a better I.D (be aware that unless it is an easy call an actual I.D will be nearly impossible...)