Greenish-blue could of course be a "blue-green algae" i.e. not a true green algae which is a plant but rather a cyanobacteria. If so, as it grows it will come to look more like a slimy sheet of material draped over the rocks rather than a green or reddish-brown carpet rooted on the rocks. Just another alternative to bare rock, the brown slime of di-atoms or the reddish/brownish/green of a green algae that you variously get in a tank depending on light conditions, nitrate levels, carbon dioxide levels, phosphate levels and so on which dictate which will out-complete the others. You tend to get what seems like a sequence of one to the next in a new tank due to general changes in these parameters as a tank matures.
I don't think it is anything to worry about. I often get blue-greens growing in my tanks, especially covering shell beds, and they have never caused any problems with the fish. After all, spirulina is a blue-green and apparently a lot of species that appear to graze on green algae (which is actually relatively difficult to digest) are in reality feeding on the much easier to digest blue-greens that coat the strands of the green algae the later passing through the gut relatively undigested (just read this is Ad Konings' new tropheus book).