Is your water source on any sort of softener system?
this isn't characteristic of any diatom variety I've heard of, it does look like a fungus though but can't be sure yet. I think the least intrusive treatment at this point would be baking soda and if that doesn't work you could try a antifungal/antibacterial treatment next but that will put unneeded stress on any inhabitants, seeing as how it doesn't seem harmful yet, my treatment regime would first be to wait a few days and see if it begins to clear, if that failed I would next try a baking soda solution, next would be to quarantine the fish and dose the tank medicinally. That just my personal regime I would try at home going into this.