Red (algae?) only under sand? What can I do?

[Mbrklynb]

Hey guys, just posting to see what this is and what a solution to it is.
It’s strange because it’s just underneath the sand where it hits the glass, it’s not on the actual sand bed or walls of the tank.
There is some on one of my white rock decors though.
Thanks!

 

[Copingwithpods]

I'd wager normal film algae lit by the reflecting light that doesn't get cleaned but I could be wrong. Can you snap a pick of it on the white rock?

 

[Mbrklynb]

The tank just hit a year a few months ago and this is really the first sign on algae I’ve had.

 

[Mbrklynb]

Anyone?