help with posible fungus infection

[marvinsae]

i've tested my water parameters, alkalinity, calcium, ph, ammonia, nitrate and everything is fine. only thing i dont have a test for is mag and phos. temp is a little low at 26 and salinity a bit high at 1.027. so my blue powder tang has a few white spots on his stomach and a few on his fins. im thinkin it's a fungal infection. how do i care for this? i heard it could be due to stress cause i just added him a couple of days ago. will it go away or would i have to quarantine him and how would i do this?