Rust Colored patches on My Yellow Seahorses

[azsooner]

I have noticed dark rust colored patches developing on my yellow seahoreses. The seahoreses are acting fine, eating, and I haven't seen them scratching. Does anyone know what this is? At first, I thought they might be changing colors. However, if that where the case, I think it would be more evenly spread over their bodies. Sorry for the picture quality.

Thanks,

Jeff

 

[Hara]

it looks like cyano to me.
Seahorses quite often will get the algae growing on them as they are so sedintary.
Do you have any of this showing up on the substrate or rocks in the tank?