It kinda looks like ich/ick. Not 100% sure though. What kind of fish is that? What's your current water tempature and parameters? That would help too because there are different treatments. Some fish are sensitive to meds, and some are sensitive to aquarium salt. U can slowly up your tempature if you have an adjustable heater but you will have to wait for others to chime in as to what temp and how slowly to adjust your temp up to that point. Sorry I couldn't be any more help