In the first image I mostly see spirorbid worms (
Spirorbis sp.). These should be harmless even if they grow on coral tissue. See
page 5, post #81.
What I'm not so sure of are the white blobs that are stuck in the zoas polyp opening. I don't know if these could be eggs of some sort.
I think they are too large for Zoanthid-eating sea spider eggs and you would see the adult animals on your corals from time to time. Zoanthid-eating nudibranchs lay spiral egg clusters so that's not a match either.
I would wait until the lights are off for 30 minutes or so and then investigate with a flashlight if there is something on those zoas that comes out at night. Expect to find various pods and maybe bristle worms, but not all bristle worms are harmless.
Another option would be an infection of some sort. While zoanthids are certainly very easy to keep compared to many other corals they still can suffer infections and die. See
page 8, post #144. The videos were taken from an infected zoanthid. In that case I would expect slime/mucus to accumulate or the polyp turning dark brown/black and shriveling away.
I hope someone can identify the white grains for you...
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