As noted above, many diseases come from poor water quality and/or stress on the fish.
What size tank is it? What other fish are in there? Is it overstocked? Do you do 50% water changes 1-2x weekly? Vacuum the substrate? CHeck your water for ammonia, nitrite, nitrate? Is the tank cycled?
I'd start with 50% water changes every day for a few days and monitor the fish. If the symptoms don't get better, quarantine it and you may need to try some meds but you'd need to research the symptoms to find the correct medicine. Id' also be careful about dosing the whole tank with meds b/c some meds can kill your beneficial bacteria as well. But right now water changes ASAP.