Diatoms?

[Mikek]

So, the orange stuff...diatoms? If so, I have read "less light and less food". Will that be enough?

 

[RockStacker]

Those do look like diatoms.
They use the silicates which usually come with some types of dry sand and rock.
Very little silicates are introduced via fish food, so the diatoms will eventually run out of food and die out.

 

[ErinCahir]

+1 rockstacker.

They'll die off with time. Harmless, but ugly.