Dragon fish mold

[r-pain]

Petco sold me a dragon fish saying it was perfectly fine in fresh water. However everywhere else said they are brackish or they slowly die. I picked this guy up about 4 days ago and the second day he had white fuzz on his head. All water parameters are normal, I test every two weeks and I tested last night. I'm returning him tonight but I want to know if this stuff can spread or not, I've already done a 50% change since I quarantined him.

 

[r-pain]

15 gallon tank with 3 Mickey platys and a fantail darter

 

[Delapool]

Not sure if that is fungal or bacterial. Both can spread but I find fungal tends to only be on one fish or so whereas bacterial can easily spread through a school or entire tank. I'd watch your other fish.