Brownish yellow Rocks

[molotov]

I understand there are different types/colours of coraline algea. The rocks I bought to seed with had a nice deep purple colour and are full of coraline. I was hoping that if I was going to get coraline that it would be a different colour than the brown.

I'm starting to think now that it could be a diatom bloom although my sand is in perfect condition. I'll try to get some pics up today and let you guys be the judge of what's going on.