Rust-colored algae? ... or what?

[HowardCath]

This stuff (see patch in the center of the picture) is spreading in various patches, including over purple coralline algae. These rocks are several years old and this is new over the past few months. Is this recognizable to anyone? Is it some undesirable algae and is there any remedy?

 

[jhutton]

dosen't quite look like cyano, i think it might be diatoms...

 

[HowardCath]

It resembles what I have seen as diatoms years ago, but it is more rust than brown and it is not diffuse all over the tank. It spreads from specifc points. It grows down onto spots on the sand overnight and my diamond gobie doesn't eat it.

???????????

 

[HowardCath]

Next clue: Looks like rust on the rocks and golden-brown in patches on the sand. It blows off the rock easily with a turkey baster, but grows back in the same spots in a day or two. Can cyano look like this?