Yes. That's normal behavior of Foxface. It is a very shy fish and when frightened will hide and turn dark or lose its color. At night mine rests at the corner of the cornerflo without any bright color. Is yours eating when fed? If yes, your fish will be fine.
You know, i haven't been home when the feeder has gone off the last 2 days so i haven't seen him eat when that happened, but i have seen him eat away at the GHA during the day. I've cut down on my lights 2 hours less a day so i have my feeder go off later in the morning when they come on but i am not home. Its a relief to read this about the foxface cause he has me worried.
May be you should try to turn off the feeder and just manually feed the fish in the evening for the next couple days and see how the fish eat.
My feeder comes on at about 2p each day when the lights are at the highest intensity so the fish can see the food better.
Here is some info from Drs Foster & Smith about FoxFace:
"The Foxface Lo, also known as the Foxface Rabbitfish, has a very unique body color. It has a mottled yellow-brown body with a dark chest. The face is very light in color and speckled on the lower half. They are a very hardy fish, and are great additions to the newly-established tank.
It may reside in a 125 gallon or larger aquarium. It is a very peaceful species except when housed with other rabbitfish. The Foxface Lo may be housed with more aggressive fish. Its predators will tend to leave it alone because of its venomous dorsal spines. Rabbitfish are generally reef-safe if they are well fed. If not, it is possible for them to nip at and consume some species of LPS and soft corals.
The Foxface Lo diet consists of a variety of fresh vegetables and undesirable species of algae. It does nip and possibly eat some soft and hard coral polyps."
FoxFace is a herbivore so you need to include some sea weed in the diet besides just flakes and pellets.
Do it gradually and never more than 100ppm per day.
I too was puzzled by the high Ca level as commercial salt mix don't have that much calcium in them. I thought may be your rocks have a high calcium content and will eventually come down. I am glad you have found out it was the testing equipment. The only Hannah test kit that I have is Phosphate Checker and I do all my other tests with Red Sea. How high are you going to drive your Ca up to? 450? Keep an eye on Alk as you increase your Ca.
Will definitely keep an eye on it. Going to bring it up to 425. Should it be more?
My Alk is at 8.5, I've been running my doser for that at about 20ml per day