Could be fungus, but could be a lot of other things as well (e.g. Lymphocystis, physical abrasion w/ possible bacterial infection, etc.). It's very hard to tell at this stage. I'd personally observe it to see if it gets better or worse. If it's not fungus and you subjugate your firefish to the stress of quarantine and medication, it might do more harm then good.
Spooky update: The fish now have more white blotches and they seem to be moving - So it could be some parasite - What is the best way to treat? Fish are in QT so I can be as aggressive or slow as needed
Are they moving to different areas, or moving in place? it could be external worms (treat with anti-worm medication) or small isopods (does not look like it in the photo though). I'm not sure how to treat isopods. If it's moving in place, it could be fungus flapping in the water. Anti-fungals like formalin will do the trick. Sometimes hyposalinity works too (slowly lowering s.g. to 1.005-1.010).
They seem to be staying in place - Just flapping - Fish do not show any sign of stress so I will continue observing while continuing my research - Quick question: I guess that i should buy a refractometer if I want to correctly measure hyposalinity?