nice tank, and a sweet little acro. those crabs are soo cool, and in the wild, they actualy protect the coral from predators such as the crown of thorns star. anyhow, i think the shrimp that might have came iwth the colony could be a pistol shrimp. in the wild, if they dont live with a fellow watchman goby, they sometimes hide out in acro colonies!
good luck
jon
p.s. what size tank and lights do you have?