its micro green algea. its quite common for newly setup tanks with new rocks(ie mostly dead rocks). the algea is feeding off the nutrients,po4,no3,etc leaching out from the rocks. it looks like the middle section rocks r leaching more. that is why its always better to start out with good cred live rocks with plenty of coraline algea so that other algea will not get a footing on it.
depending on how well you keep your water par, your rocks green algea will either continue to get hairy or cease to exist and be replace with coraline. common color of coraline r purple,pink,red, orange.
your water is clear is probably cause u r using filter pads to filter your water. this may not be good for a reef tank. crystal clear water does not mean your par(ie-ca alk po4 no3,etc..) is good. get some good test kits to get a better idea of how your water is to avoid problems down the road.
btw: having the wrong spectrum of lighting will also promote that type of green algea.