I believe the reason it appears to be growing under the substrate is because I moved my live rock around a good amount and the substrate got overturned but If it isn't algae idk what it could be here's a couple more pics
that could be cyano. you can try increasing flow in that area, make sure your not over feeding and some extra water changes. hermits may or may not help. is this a newer tank and are you running a refugium?
It is a newer tank 2 months old I have a in tank refugium in one of my rear chambers with chaeto only with a 10w submersible light in a custom caddy I have a nuvo fusion 30L