I don't see evidence of a low iron problem.
Yes, the coral can regrow/regain color, but...answers to the questions below will help track down the problem:
- I know it's hard to tell without removing the coral and examining it with a magnifying glass, but are those white spots merely bleached or has the flesh of this Montipora actually been eaten away?
- What about the red Montipora, partially seen in the pic, are those coral showing similar signs?
- Are any other coral in the tank showing unusual signs?
- Is there an Angelfish, a Blenny or possibly a Damsel in this tank? What kind?
- Is that an anemone to the left, next to various Zoanthids? Did it go on a "walkabout" and sting the green Monti?
- Hogie asked but I didn't see an answer, Did detritus settle and then recently get removed from the bottom of the Monti cup? This would be a separate issue from the white spots up the slope.
- One important parameter not given above is temperature. More specific to this small tank which, when I visited a few months ago, was located on the kitchen counter, What is the daily temperature high point and about how long does that temperature last?
Lastly, I don't quite understand this comment found at the very last of the OP, please explain,
"It's still growing like a weed but I love my green color. Also I've lost all my green color in my grafted monti."
Looking forward to reading the answers and helping you find a solution.
Aloha,
Mark