White spots on heaters...

I have 3 five gallon tanks that are all thriving nicely. I have the
same heater in two of the tanks and both heaters are covered in what
could possibly be fungi. Only one of the heaters is kept on at all
times. The other one hasn't been on in almost a week because the room
the tank is in is warm enough to keep the tank at a steady
temperature.

At first I though that the white stuff was mineral deposits from out
hard water but the third heater shows no signs of white spots (it's a
different brand). Does anyone have any clue what it might be?

One of the tanks has 3 platy fry and an adult platy while the other
tank has two neons and two black tetras.