whats happening to my Favia

[MitchM]

The randomness of the white spots make me think that the hammer may be responsible. I've seen sweeper tentacles from both my Favia and anchora (hammer) reach 6 - 8 inches in length.
My Favia is about the same depth under 400w MH and is one of my hardiest corals.
Have a look during the night for sweeper tentacles. They will still probably come out even after you move the Favia.
3 weeks is still a relatively new addition. I have found that the corals eventually get used to one another. If you run activated carbon, they may constantly be at war.

Mitch