These clown gobies are commonly found in wild SPS colonies. Small ones can be tiny and just grew up in your tank. If you get another one, they will breed in your tank. Neat little guy. This green one tend to be larger than the yellow one. When they make a nist, they will strip tissue off of the SPS colony and lay their eggs there. They use the same nest site time after time so the damage is minimal, especially in a large tank like your's.
Minh