What's on my plants?

[mim]

I have a low tech tank and seem to get this funk that grows on my plants. What is it? It rubs off but it always comes back.

 

[megi-lin]

I don't know. I always assumed it was algae. I get similar stuff on my tank walls and heater. Following!

 

[flchamp89]

Seeing how its brown and rubs off comes back. Probably diatoms. Google away