[Skim]

Coral was colourful and started to turn brown, most of the time it mean's Phosphate and build up of Organics.

This is what I would do. Pull any GFO and GAC and go get some Seachem Purigen and couple of the Seachem " The Bag " put it your sump or better would be Canister or a AquaClear Hang on. This stuff removes Organics quickly and you should see things get better and your Phosphates should drop also.
The other thing is make sure your skimmer is working at 100% clean the neck and make sure there is no blockage of the Air supply.
Do not dose or add any 2 part solution if that's what you use for now as it also adds Trace Elements that could feed the Algae.
Good Luck!

Mike