Acans receding - looking for suggestions on possible causes

[gregzz4]

Bye the way, thanks for the input guys

So, anyone have any other thoughts on why my acans decided to recede ? Just a random head here and there
One colony has shrunk 50% around the ring - as in from 1 to 8 o'clock, another the head just vanished, and a third is going from the outside-in
Nothing the same between the 3 different corals

They were dipped and inspected last friday and nothing showed up in the dip water
Plus, the dip gave no improvement