Concerned about my war coral

[ConnivingPigeon]

Hello!

I was gifted an established reef tank by a friend, and I'm still learning the ropes. I'm a little concerned about my war coral, over the last few days, i've noticed it's been .. puffy and the heads have shrunk down a bit, when I was changing the water earlier, I noticed a "bubble" of coral on the back had come loose and I just now noticed that the bottom edge has some white to it. Is this a natural thing because that edge is shaded from above? I checked the water params yesterday, and everything looks good except the nitrates are high (40) and the kH was low (7), which is what caused me to do an extra water change.I just so happen to have a picture from a month ago for comparison, if that helps

Thanks!

 

[ConnivingPigeon]

Actually, you can kinda see the bubble in the "before" picture in the top-left-ish of the photo, so I guess it's been building for a month

 

[ConnivingPigeon]

Hi again, I actually got my camera around the back of the tank and it looks pretty gross. What am I looking at here? I've done a couple water changes since last week, so I'm hoping whatever it is that's out of whack will get cleaned up, but I'm worried for him