My First Saltwater Tank! 46gal Bow Front

Can I get some help please?? I've noticed the last two days a brownish/reddish like like stuff developing on the sand. Can anyone tell what what it is? More importantly is it bad or good? Cyano maybe? (Hope not). If it is bad how do I get rid of it? Thanks for your help

How long has the tank been running? It looks like the beginning of a diatom bloom to me. Which may get worse before it gets better but they will die out, it's a phase all tanks go through.
Diatoms feed off of silicates in the sand/water/rock. Once they have consumed all of the available silicates in the system, they will very quickly vanish.

#New tank syndrome

It's been a while so I thought I would do a little update. I converted from T5's to LED's. Got new Kessil A360WE lights with gooseneck fixtures. They really make the tank pop! Taking time to acclimate them since they are super powerful! Also added from more LR and re did some Aquascaping!