Closed up zoa colony - need help

Hello reefers! I've got a coral problem.
I have a rock with 3 types of zoas on it: green, reddish brown and milk chocolate brown. While the green and chocolate are doing fine, the reddish one closed up a few weeks ago and doesn't open anymore. I've had these zoas for over a year, all were doing well. 2-3 mos ago the tank went through raised nitrate period and many panic-y water changes. My water levels are ok now, except for a slightly high Cal and slightly low Mag, so I've started dosing Mag a little. I don't think that water parameters or light could be the culprit, since other zoas are fine. I do have a lot of diatoms and some brown spiderweb-like and hair-like algae that I'm fighting - could this algae be suffocating zoas? There is also a small colony of zoas close by on the same rock that seems to be covered with brown algae and doesn't open anymore... And I've noticed some white spots on the zoas, which I first thought were sand, but could it be that the zoas are sick? Please see the pics below and share your throughts or experiences.
Thanks!

p.s. I have researched this issue for a while and have seen something on the chem wars between zoa colonies - could this be what's going on here?

 

No ideas? Sorry if the pic isn't clear, didn't have much luck with another shot but will try again

 

White spots? Have you researched Zoanthid Pox?

This algae you speak of could it be Dino? Dino can kill Zoanthids.

And you have ruled out all the usual suspects:

1. Zoanthid eating nudibranchs.
2. Sundial snails.
3. Zoanthid eating Asterina stars.

 

Thank you Corailline, will have to look into these

 

Happy to post I beat the issue down. It was an algae hurting the zoas, though I couldn't figure out whether it was dinoflagellates or calothrix. I used a lighter version of Coralline's suggested treatment: had the lights limited to a few hours or blue light only (LEDs) and daily turkey blaster on the algae (manual removal), as well as less frequent wc with just minimal amount of water being changed. And I've added 1 nerith snail - those are supposed to lay unsightly eggs but I'm hopeful 1 can't. I think the manual removal was the most important part here. Anyway, thanks again Coralline!
I was so happy to win this battle, I've even rearranged the tank!

 

yay! that Corailline, she's the bomb! Tank looks great and most importantly, your zoas look awesome! Glad you beat your culprit!