Monti damage

[ReefRookie22]

Any ideas on what might have caused this damage to the front edge of my Montipora? Has happened over last 48 hours.
Not changed anything recently. Parameters pretty stable. Slight Alk drop to 7 but brought up again.
I think the same thing happened to one of my chalices recently too. Looks like something had eaten the soft tissue (bacteria, urchin, hermit crab?).

 

[Gumbies R Us]

What CUC do you have in your tank? I have never had my urchin bother any of my coral

 

[ReefRookie22]

What CUC do you have in your tank? I have never had my urchin bother any of my coral

Few Nassarius, trochus and turbo snails, 3 hermits, one strawberry conch and one red tuxedo urchin.

 

[smacrophylia]

Looks like stinging from another coral potentially or monti eating nudibranch. I had a rogue blenny for a week that would suck the pigment off of one of my Monti’s. The damage the blenny caused was bigger “bite marks” though. Look at it very closely after lights out. The monti eating nudi will be visible to the naked eye. All just guesses but good luck to you!

 

[ReefRookie22]

Going downhill fast!