Cyano is my guess. It tends to develop on the bottom where diatoms tend to grow on the glass as well as everywhere! LOL! But if this is a new tank, diatoms is a real possibility. The answers to tupes' questions will be very helpful in a solid ID.
Cyano is not an algae, it's a bacterial bloom. You can suck it out with a syphon for starters. It also doesn't develop well in areas with more flow. In fact, I started putting my rockscape up on an egg crate platform with pvc pipe for legs that set the platform an inch or two above the sand. That way I get better circulation of water over the sand bed with no rocks to block the flow. It also exposes the bottom of the rocks so beneficial bacteria from the nitrogen cycle can develop. The critters that like the sand also have that much more square footage of sand surface to play in. The area under my live rock platform was covered with sponges and the sand under the platform was covered with small feather dusters, spaghetti worms and a couple of small mollusks (baby giant cockels).
It helps us to identify the problems if you post the results. All those things are contributors to algea growth and it helps us to tell you how to get rid of it. I know testing isnt the most fun thing to do but it is necessary
New tank I have is starting to do the same about to do a water change to suck it out everything is testing normal for New tanks but I just haven't got my po3 test kit yet. I'm figuring it's that but no way telling and my job paydays are Monday. But hopefully I can control by doing a little mantaince until I get my test kit. Happens a lot with new tanks have battled it before with old tank in the past. Next buy is a uv sterilizer for sure.