Heres mine, i have been blasting it with flow and light and it has got very purple, almost to the bottom of the whole peice. This pic is 2-3 months old but its the best pic i have of it. Again this pic does not do the peice justice as of now.
I'm going to go against the grain of most wild guesses posted....and go with acropora eating flatworms (AEFW for those that like acronyms).
My reasons without any proof:
1. These buggers love tricolors! I have seen them kill a tricolor colony in less than 2 to 4 weeks. Other infected corals take months to show any subtle signs of problems.
2. You obviously know how to keep acropora, yet your tricolors are bleaching from the bottom up. Very atipical of AEFW.
3. Since you didn't mention it, I'm going to assume you didn't quarantine these tricolors, or any of your acropora frags...since most reefers don't!
To get proof of AEFW:
-Examine the base of the colony looking for eggs like this: