Green stuff in my tank?

I have a 29g tank, with a few mollies, 6 platies, 3 guppies, and some fry. Today i noticed green stuff on my heater and on my tank, i have no live plants, all are fake. I was thinking it might be the flakes that im feeding them. I am feeding them tetra tropical flakes and on last question can a keep the temp at 80 deg f with my fish and for my fry and can i add salt.