Algae always grows on bare surfaces easier. I would say just because it's new, there is a bit of algae, but it should go away. Is this a new tank, or did you just change all the substrate? If it's new, it's just cycling, if it's a cycled tank and you changed all the substrate out at once with new substrate, then it might be cycling again and this might just be an algae bloom. Did you check your parameters?