Brown Goo Substance On Rocks

I have noticed about a month ago this brown goo substance on these decorative white marble rocks.  I usually clean it off but I decided to keep it, because some of my algae eating fish ate it, sooo I'm guessing it is a form of algae maybe? It doesn't affect my tank at all I don't think, it just looks nasty all over my white rocks in the front. What do you guys think?
 
You can sort of see a little bit of it in the very bottom part of the white rocks here.

The rocks are always blown out in my pictures

I can't see anything, sorry.

Is it on the white rocks only? My first guess would be a diatom bloom, but it would be everywhere, not just secluded to specific areas.

I can't see anything either, as the white rock is reflecting the light.  Which reminds me, why white substrate items are not a good idea.  You have very nice black substrate, I would suggest removing the white rocks.  They will be covered with algae, whatever the substance may be that is there though we can't see it.  B.