from http://www.saltcorner.com/sections/zoo/1024frameset.htm (Gobies) :
"Trimma
This genus consists of 39 identified species, with many still waiting to be identified. All are quite small, i.e., less than two inches (5 cm) in length. One male usually dominates a group, with all others being females. If the male dies, one of the females will become a male. Should a larger, stronger male become the leader of the group, the less dominant male will return to being a female. And do so quickly, sometimes in as little as four days! All species are peaceful and in the wild feed upon zooplankton and benthic invertebrates. In the aquarium, they will accept meaty foods such as brine shrimp and mysis shrimp. They are perfect inhabitants for small nano reef systems.
T. anaima
T. annosum Greybearded Pygmy Goby
T. avidori
T. barralli Red Pygmy Goby
T. benjamini Redface Dwarf Goby
T. caesiura Caesiura Dwarf Goby
T. caudomaculata Cave Pygmy Goby
T. cana Candycane Pygmy Goby
T. emeryi Emery's Goby
T. fishelsoni
T. flammeum Orange-spotted Pygmy Goby
T. flavicaudatus
T. halonevum
T. hoesei
T. macrophthalma Flame/Big-eye Pygmy Goby
T. milta
T. naudei Red Pygmy/Rubble Goby
T. okinawae Okinawa Rubble Goby
T. rubromaculatus Red Jewel Goby
T. striata Stripehead/Red-lined Pygmy Goby
T. taylori Yellow Cave Goby
T. tevegae Blue-striped Cave Goby
T. unisquamis Cheekscale Dwarf Goby
T. winterbottomi Winterbottom Goby"
Your looks like perhaps T. anaima or T. benjamini (Redface Dwarf Goby)