What is Wrong - Acropora white tip

[Bolek]

hi, I am searching for advice, what is the problem + solution to my acropora problem - where one of the tips is getting white (from the top), while all the others are fine. The whitening is spreading, it started small and over the course of 5 weeks expanded to current state as seen at picture.

I have many other acropora, no other is showing the same issue. No changes in water parameter in last month (Kh7.5, Cal420, Po4 0.05, No3 5ppm, 250par)

Took a night photo, so that it is visible that the coral also on the white tips is living with polyps extending.
Any idea what is happening here and what could be the solution?

 

[blaxsun]

Anything possibly picking at that during the day or night?

 

[dadnjesse]

New growth will have white tips, but that looks like a lot of white it's hard to tell from that photo.

 

[Bolek]

No, no fish picking on it. Here is a better picture during the day. Any ideas?