I would suggest starting with a tds
meter and testing your source water. I would also think about getting a phosphate removing media such as GFO
Did you start with dry rock or was it live. If it was live rock, where did you get it? I remember when I first started out I added some live rock from the LFS
that had a little HA on it. In my newly established tank it just took off and grew like crazy.
Also, how did you cycle the tank?
I guess overfeeding is out of the questions because you only have a cuc.
Its very common to get algae in a newly established tank so don't get too discouraged. Main thing is to figure out what is causing it and then move forward from there. Patience is the key.
Try to manually remove some of it from the sand bed and if the rocks are bad, just try and scrub them off a little at a time. Make sure you rinse the rock in RO
water before putting back in tank.
Good luck, and like I said, don't get discouraged.