What is this ?

[David]

Hi: I have my tank for about a year now. Lately I see these brown/rusty looking stuff started to grow. Please advise how do I get rid of it and what they are. Thanks in advance.

[Der_Iron_Chef]

Looks like Lobophora....a type of plating algae. I have it in my tank and it hasn't caused me any problems.

[Redrover]

Me thinks it's a type of incrusting brown algae:

[christyf5]

Brown wafer algae. I have it too. Scurge of the marine world (except for grape caulerpa, and dinoflagellates). I have no idea how to get rid of it, I just remove it manually once it gets big enough you can peel it off in mostly one piece.

I think reef_raf had this as well. Not sure what he did to get rid of it.

[Ephraim]

Peel it off? I love the look of it in my tank, once it grows in really big it looks like miniture monticap.

[christyf5]

Well yeah at first it does look kind of cool, then when it takes over and outcompetes your coralline. I'd rather my rock not all be brown, it kind of detracts from the rest of the tank and gives it a "dull" look. JMO.