Coral bases turning brown?

[MissSquishy]

I have three corals that are turning brown.
A green slimmer.
A knuckle coral.
A galaxea coral.

The tentacles and polyps that come out are not brown, but their skeleton, or base is brownish. If i rub my finger on them it seems to come off, but they don't like this of course and I'm sure it's not the best option.

I think it's killing my slimmer, so i'd like to figure out what this crap is so I can get rid of it. I'm trying some coral snow stuff here and there, but not sure if it's targeting the problem here.
My tank stats are.

Ammonia:0
Nitrate:0
Nitrite: 0
Phospates: between 0 and 0.25 (Hard to tell with those things)
calcium 420
Alk: 9.8 (a little down, working on it.)
Mag 1400
Salt 1.024
tank is 3 and half months old.

test kits are between red sea and API

lights are 2 250 watt halide
1 t12 acentic

Any help would be great!

 

[ClownReef®]

Flow? Pictures would be helpful.

 

[MissSquishy]

Flow? Pictures would be helpful.

Hey! Thanks for the help!

I have plenty of flow with two 1321 gal/hr pumps, and I turn on my 2k gal/hr Korella.

I took a picture of the Knuckle coral and the galaxea coral as I had a friend put my slimmer in her tank because I felt like maybe it was something in my tank and it's killing it.

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[NaturalReef]

Your phosphate may be higher than the kit is reading.... Are the rocks covered in brown Algae or it that just the pic?