That is cyanobacteria in the sandbed. Typical to see in tanks fresh out of cycle, but it is an indicator of excess nutrients in the water column.
water in top offs and water changes, feed minimally like 2-3 times a week, and siphon the cyano
out during water changes. It can take some time to get rid of...they make products that will make it vanish, but doesn't resolve the issue that causes it. It will then return without addressing the underlying issue. That and adding chemicals to your tank should be a last ditch effort.