Algae?

[Zac]

Trying to figure out if this brown stuff is diatom algae or something else. I’d like to add shrimp soon. Attempting a plant only cycle but I do have pond snails.

[AndreaW]

It looks like either mulm or diatoms (brown algae) to me. Both are fine and normal.

Diatoms rub off and will be on surfaces that get light (so you won't find it under an edge of a rock, etc.), but mulm will settle on any surface it lands on and will float if you disrupt the water around it. 

If you have a lower water flow, and pond snails, you can expect mulm (mostly snail poop at this point) to settle throughout your tank. Pond snails love diatoms and will leave tracks through it as they munch on the go.