Diatoms pretty much cause a brown discoloration on the sand itself. But it Almost looks like cyanobacteria which is more gooey like ontop of the sand. Cyanobacteria is easier to remove but indicative of excessive nutrients/nitrates&phosphates. Increasing water flow to the areas of cyano
will help prevent buildup and there are additives to get rid of it. But you'll still have the cause to deal with.
If it is diatoms tho, your cuc will get rid of it in time and is very normal for most new tanks. They eat it at night and it comes back in the day. Will last for a couple weeks to coupe months.