I can't tell anything from that picture to be able to say what that fish might have. Have you looked through the diseased fish section? Do you see anything that sort of looks like it? Beth posted some great pictures in there.
As Meowzer mentioned, get some vitamins in the food, my personal preference is VitaChem, but many people also like Selcon.
You still haven't posted your parameters. When someone asks you for parameters, they're asking what your water readings are such as nitrates, ammonia, nitrites, pH, alkalinity (aka Carbonate Hardness), calcium level, water temperature, and salinity. If someone asks you what setup you're running, thats when you say what type of filter, media (such as carbon), power heads, etc you have.
IMO, frozen brine shrimp is GARBAGE. It should only be fed as a treat to your fish, especially if it's plain brine shrimp and not even spirulina brine. That fish needs a better diet, and fast. You need to feed mysis shrimp, and mix it up with other meaty foods as well. Meowzer mentioned some excellent foods, but mysis should be a staple in their diet, especially if they're sick.
IMO, your fully stocked as far as fish go in the tank and when you move to a bigger tank, wait awhile before you load it up with fish or you'll have major issues on your hands.
Now, back to my original questions:
What are your water parameters?
Have you looked through the diseased fish section of the forum?
