I had 6 starving emerald crabs and 1 small blue tang with no algae at one point in time.

I have a Royal Gramma which sleeps lying in the same crevice in the rocks every night. She has occassionally had a chunk out of a fin. Would she be safe with an Emerald?

I can't answer that question. But I have 2 sharknose gobies now for over a year without no problems. They live inside rocks. And my blue tang is still there with no problems and I'm not regularly losing fish.

As with any living animal (including humans) there are gonna be those bad apples. I wouldn't read too much into cases like these unless they start to become more common than not.

Im not convinced that these were Emerald crabs. He said he may have at least 6 of these in his tank. He didnt say he bought them at all. 6 Emerald crabs just happened to be on his live rock when he bought it... come on. What I can see is them being another type of crab just feeding on dead or dying live stock. Id say your safe buying an Emerald crab, but it is hit and miss as to weather or not they are going to eat the algae you want them to.JMO

Emerald Crabs are omnivores
 

I would my Emerald crabs in a reef, Scotty. I have seen them catch ill fish and smaller healthy benthic fish such as gobies, blennies (Never healthy tangs etc. thats crap), but as well they can also prey on corals. I have seen them attack SPS a couple of times (It is rare, but happens) One of my clients has had an emerald crab from the beginning in his 120gal reef 1.5 years ago, and I put a Hydnophora frag in there and in 30 seconds the Emerald crab was sitting there scraping away - Last Week. It destroyed the stony coral frag quickly, and then tried to move onto some other SPS frags that had been in there for months. Needless to say I removed the little rogue. And no, there was no identity problem here, though there frequently is at Pet Stores.

They will eat anything if it appeals to them. They are absolute Omnivores.
They can snack on Valonia spp. Algae (Bub.) but they rarely clean the entire tank quickly.

Destroy the Valonia with a syringe full of near boiling hot water. Remember to fill the syringe half full, then initially draw back some Valonia fluid into the syringe then pump the hot mixture back in. Tedious but effective, and you don't even have to burst the bubble.

Good Luck.
Stephen