How to get rid of these red stuff?

[Spyd]

It is cyano. It does just show up when the nutrients get too high or not enough flow. The seahorses and corals all exhaust nutirents into the water and if you do not have enough flow, it settles on your sand bed.

Since you cannot increase the flow, siphoning it would be your best bet. It can also occur if your light bulbs are getting too old. Do you run GFO or anything like that?