So in that picture, on the base of the dorsal, is that one, or a grouping of them? If it's a grouping right there, given your measuments you provided for an individual..... it could be a case of Black Spot. It's most often seen in Tangs but don't be fooled, it can and does infect a whole host of others. It's a parasite, yes. But not a lice, that's probably why you couldn't find anything close to what you were looking for. Black Spot disease is actually a flatworm - turbellarian flatworm. If it is infact flatworms, there are a few things you can do to get rid of them and save your fish, but what I'm about to tell you is not something that's going to make you happy - just warning you. These parasites live in the substrate - which just made your goby all that more of an appealing host. At the larval stage, they feed on detritus matter in the substrate and lack pigmentation, being almost transparent - which is why it's not a parasite you can look for until it finds it's host in your fish. Once grown is when it seeks out a host, and then proceeds to feed off it's tissues and blood - this is what gives it it's deep brown/black pigmentation, and this is when the aquariust usually first realizes there is a problem. And yes, they move around. It's life span is actually pretty short, and it generally feeds for up to a week on the host fish, and then detaches it's self and falls back into the substrate... But this is where it gets gross. utelizing asexual reproduction, once the adult is done feeding and detaches, soon after, it's body literally ruptures and out comes a whole group of new flatworm babies. And so, the cycle starts all over again - in larger numbers this time. It's important to note your fish's behavior when infected and trying to cure this plague, because though it's not a fungi or bacteria, it can still proove lethal, resulting in lethargy, color blanching, loss of appetite, rapid breathing, and a very high potential for secondary infections at the wound sites.
To treat it, I'd recommend an immediate freshwater dip of all fish infected, and if you want to be doubly sure - a formalin bath as well. But don't do that until you read up on it, or ask one of us how, because it's not something to just prepare and dump your fish into. Being that you just purchased these fish, you could get lucky and the worms might not have seeded your subtrate yet as they are still attached to their host. If that's the case, it's the best case-scenario, and I'd remove those fish immediately and get them in QT otherwise you are about to have a battle in your tank which could take months to clear. But because it's always better to be safe rather than sorry, I'd move all the fish from that tank, do a freshwater dip on the two notably affected, followed by formalin (if you choose), and put them all in a QT, leaving your main tank void of all fish. Even though the adult worms don't live long, it's impossible to say with all certainty that they haven't ruptured and your substrate isn't crawling with larve. Because of that, it's imperative to remove all hosts from the tank, to prevent the larve from fully maturing and reproducing. If the main tank is infected, and the worms aren't completely erradicated, reinfection is bound to happen, no matter how effectively you treat and cure the fish themselves. Unfortunately because the worms in their larval and juvenile stages can survive for several months without a host, it's not an easy thing to do to make sure a display tank is really safe, and can be a major headache. For right now though, you need to act, and god's willing, you may be able to save yourself from having them deposited into your sand considering they apprear to still be feeding.
oh yea, and remember to stop in and thank your supplier for being the most uneducated doofus, and selling you flatworm infested fish, and let them know they've got an epidemic chillin in their tanks as we speak.
