Brown/rusty stuff on rocks

Not sure what Aptasia is or Cyano really, but I have seen this on some peoples rocks before. I have been using Purple Up for about 2 weeks and using my new T5 lights for about a day or so less than that. My canister filter had a piece broken so that has not been used in the last 5 days. I have been starting to get some nice purple showing up, but then today BAM! (like Emeril) I have this brown/rust colored stuff on most of my rocks. I don't like it and I want it to go away. What is it and what do I need to do?

I use only RO and RO/DI water.
my params are pH 8.2, Ammonia 0, Nitrite 0, and Nitrate 15 (checked by LFS today).

I am thinking it is diatoms, but not really sure. It is only appearing on my rocks and has spread fast.

Can you take a picture? You say rusty which could be cyano but your tank is too new for that...i think its your first sign of brown algae. That usually comes first, then green then cyano.

Welcome to our world and good luck. So far you have experienced all the beauty & fun part of tanks, now its time you experience the pain and suffering involved with tanks

what can I do to get rid of this problem. Is it because of my lights? I thought algae happens mostly because of people using tap water instead of RO.

It's diatoms. And there's nothing you can do about it.

For real. It's a normal part of having a new tank. You'll get diatom outbreaks now and then. They'll seem to show up overnight, and they'll disappear just as easily. As your tank matures it'll happen less and less frequently, but there isn't anything you can do about it except have some cleaner crew to keep the sandbed stirred up, like nassarius snails.

Ah the start of the dreaded "new tank syndrome"

the thing is that it isn't in my sand bed at all. Maybe because I am having powerhead difficulties that keep blowing my sand in big dune piles no matter which way I position them.

Hell jsut aim them at the rock if they are doing that good on the sandbed.

good point. I actually took a toothbrush and rubbed some of it off. Hopefully that sends it to my filter and doesn't help it multiply more.

Just wait for cyano. You'll have a blast dealing with that one...