growth on fish

???? Help? Please? It’s white and fuzzy. I’ve seen this on a couple other of my fish but MUCH smaller and it typically kills them. I don’t know what it is or how it spreads. So. Help 🥺 the white fuzz always appears at the anal area and no where else

My partner has noticed this morning a strange lump/growth on the side of one of his female platys. She’s still swimming around happily enough and eating, but we’d just like to know what this is and if it’s something that needs treating (as she lives with a number of other fish). Tank is 120l...