White spot on montipora

[hsnydn]

Hi,

My green montipora having white spot on its tissue. The white part is only comes out at night. During the day, it is totaly not visible. I dont see any nudibranch or another living creature around there. What this can be? I dont want to deal with nudibranch. Hope it is not.
It is very similar to the white spot on one of my favia. I am not sure what it is as well. Maybe sweeper tentacles ready to fight?

 

[T-J]

Any update on this? Have you tried blowing it off?

 

[hsnydn]

Nothing changed, Dipping didnt help. Actually coral is healthy. Once blow, the white spot shrinks itself.