this is under Red light.It looks like you got more going on there than diatoms. I also see some red algae , similar to BBA on the rocks.
With diatoms you have several approaches available to you. One is to move the system towards higher water available nutrients that favour plant, green algae and bacteria growth. In time these will displace a large part of the diatoms . Add diatom eating creatures like snails, shrimps otos. Move to a soft acidic water system. Like with many algae when your system is growing healthy plants the algae tend to go away.
Such BBA growth on rocks is usually a sign of poor filtration or maintenance of the substrate /tank.