Please help me identify this algae

Take a new toothbrush and twirl the algae around it. If it pulls away like spaghetti on a fork it is probably string / hair algae. Trim away the most affected mosses while the filter is off, syphoning up the rest. Then try keep on top of it with a toothbrush and syphoning.

If that doesn’t keep it in check you could start dosing Seachem Excel, which contains an algaecide. If that doesn’t work you might have to look at other treatments like hydrogen peroxide, or citric acid (the latter not directly in the tank) or ditch the mosses altogether.

 

Also, unrelated, just keep on tracking your pH as CO2 issues could cause a rapid drop in pH.

Do you know if your water is soft (low KH and GH)?or hard? If it is particularly soft like mine, it will be more vulnerable to pH swings and a bit of crushed coral might help to buffer.