I have had some success treating sick fish, but not a lot, so you will be better off with advice from someone with more experience. But in the event I am the only one who chimes in, here is what I would do.
Based on the attached chart, my best guess is a fungal infection. It is hard to diagnose these without experience. It could be something else. The picture is not great. Maybe it is Lymphocystis.
I would put the fish in a 10G hospital tank. This can be very difficult and very stressful for the all of the fish. Often requires taking the rock work apart. Maybe the goby has a hole that will make it easier.
If I thought it was Lymphocyrus, I would Keep the fish in really good water quality with places to hide in the hospital tank.
Otherwise, assuming I was pretty sure it was fungal, I would try Kordon Rid-Ich as it has both formalin and malachite green.