Happens to mine occasionally. I usually do an extra water change and it fixes it up.
I don't think it has anything to do with spawning or a growth spurt. I think when it is open all the time then suddenly it closes up of course we all take a closer look and we notice all the new growth.I think that has happened over time but you just don't see it with the polyps inflated.
Nice piece! Is it an acanlord. looks like the type I have.