What is it?

[dmc13]

I have been struggling with this algae for approx a month. The tank has been set up for over a year and I use 4 stage RODI water mixed with red sea coral pro salt.

It started off as just a dusting on the sand and live rock but is now forming hair like structures.

 

[BigJim]

I can't say for sure, but it looks like dinoflagellates. You may be in for a battle, but you can get rid of it. The link below should be a great resource for you.

https://www.reef2reef.com/threads/dinoflagellates-–-are-you-tired-of-battling-altogether.293318/