Help!! Weird white stuff in sump and corals suddenly dying

[CampAquarium]

We just got back from 10 days away to find this weird, thick white substance in our sump (pics below). It's pure white, thick and is mostly on the display drain tube, and on the bottom of the refugium. There is some in the crevices of the live rock that in the refugium too. It blows off in strips or mats easily with a baster, and is soft and sort of slimy to the touch. There is brown stuff underneath and around it. Can anyone help figure out what it is? And how do we handle it? I am very worried. It certainly does not seem good and there is quite a lot of it.

We've had a few previously-healthy corals suddenly die recently, seemingly without cause, so I'm wondering if the 2 are connected. We lost a huge setosa, two torch coral heads, and currently a plating monti and an orange lepto are dying back. They are all in different locations in the tank.

Our parameters are slightly off from being away, but I don't think anything is too extreme:
Temp: 77.4
Salinity: 1.023 (looks like our ATO flooded the sump, currently doing extra salty water changes to raise this)
Alk: 8.6
Ph: 7.9
Nitrates: 3.8
Phosphates: 0.10 (looking to lower this with water changes too)

 

[MyFirstCar]

No clue what this is, but I've also come back from a vacation to this stuff in my filter socks and a tank that's worse for wear.

 

[Rusty_L_Shackleford]

With mystery problems like this, water changes and carbon are your friend.

 

[CampAquarium]

No clue what this is, but I've also come back from a vacation to this stuff in my filter socks and a tank that's worse for wear.

What did you do? Did you remove it? Water change? Dose or change anything chemical? And did it get better?

 

[CampAquarium]

With mystery problems like this, water changes and carbon are your friend.

Yeah, we already did one water change today, planning for another tomorrow probably. What about removing it/siphoning it out?

 

[MyFirstCar]

What did you do? Did you remove it? Water change? Dose or change anything chemical? And did it get better?

Honestly my tank had a number of problems that needed fixing at that time, in the process of cleaning it up/getting it back on track I got rid of it all. Did suck it out during a water change as well though.