The reddish stuff?
Based on the "new" look of the rock, my first guess would be Diatoms, second would be some sort of algae.
Both are things that a new system will go through during the first few months ... neither bad or good, just part of the system stabilizing.
My third option would be possibly the start of Coraline Algae... less likely than 1 or 2, but certainly a possibility.