Red sand?

[AlexRoerden]

What is this? And how do I treat it

[Mother of fish]

I don't know what it is. Just remove it and hope it's nothing that spreads. It could be tiny fragments of the red rock?

[LaloJ]

The brown algae in the foreground on your substrate is diatom algae, is your tank cycling? The purple area of your sand I'm not sure if it is cyanobacteria or coralline algae, I'm not even sure if it can spread through the substrate although in glass it does. I'd bet on cyanobacteria.

[Sniperhank]

That is cyanobacteria. The starts as a dusting. You see it in low flow areas of our systems where there are nutrients present. Same thing as red tide. Increase flow to the area. If it gets a large slimey coating, you will need to start to address the nutrients in the water column.
That is unless this is a newly cycled tank and then it is completely normal. Turn out the lights and finish the cycle.

[Reefstark]

Add more flow and it’ll clear it right up try too steer clear of any chemicals too treat it