Some type of goby - pics a little fuzzy but could be a scissor tail. If your tank is covered and you are missing a fish that has a tendancy to be a jumber you may be suprised to find it in your overflow compartment. This has happened to me. Thank god for the copepods that kept it alive in there. Now the scissor tail I have is always out on the open.
I think it despises the darker spots within the tank!