Anyone know how to get rid of this?

Already tried BlueExit which has done nothing.

Go do some research before implementing this but I seem to recall blackouts - for many days without peaking were the answer here, plus good old manual removal of course

Hi mate ,
Reduced lighting hours sounds good idea , manual is pain in the arse believe me , been there ,
Depending on how much and fast it grows sea urchins are great for gobbling it up and keeping it under control , lawnmower blennie are good but can be picky what they eat , think it depends on how much time you have to spend on you’re tank and if you don’t mind the manual scooping to clear it up ,
I’ve gone over to the sea urchin on a trial to see how that works , they’re inexpensive quite readily available, they climb the glass so help with keeping that clean too , best thing I done , and it’s another critter to view and watch with all its little arm things waving about .
I’ll send a picture of mine ,
Dave
 

Not so good in freshwater tho