It's a common Hawaiian Reef Lobster. It's worth about 30 bucks, maybe a bit more if it's huge. They're not that aggressive, just don't put them with small fish.
The biggest problem with these type of inverts in marine tanks is people don't feed them specifically. They expect the lobsters to just scavenge, which makes them starving and more likely to attack fish. Feed them and they rarely bother anything. There are also two similar species, the Purple and Debelius Reef Lobsters, which are better suited for a reef. Pretty easy to keep for a marine critter...