Diatom algae. Not to worry. Next you will see other colors of algae and stringy worms.

If your sand was prepackaged in store bought bags claiming "live sand" I would post a thread to see if you could get a back of actual live sand from your fellow reefers on MAAST.

I give you a small bag of some to help seed your tank. I have a 2 year old sand bed that has all kinds of things in it.

PM me if you want some. It will just be a small bag but it will help add some live stuff to your current sand.