what kind of algae is this?

[lucano]

Hello, I have these algae growing on my sand and I dont know what king it is (i'm a beginner). I had tap water in the tank but i've gradually changed over 70% of the water to ro in the past weeks.
I'm not feeding much, I've added 3 powerheads (55 gallon tank) and I've tried taking it out manually but didnt work.
I just noticed that my emerald crab is eating it. Is it a good algae to have?

Thanks

 

[lapin]

Looks like some sort of green cyano.