Depending on how often one has to clean/wipe the algae film off their glass; is a good general indicator of the nutrient levels in the tank. Generally, if you only need to wipe the glass once a week, then I would say your nutrient levels are very low etc. Hope that makes sense. Its like a visual clue as to the health of your tank, without actually looking at test kit results etc.
I dont have a blue Milli at the moment, still trying to track one down.
However, I will tell you this:
It looks like from the photo that your Blue Millepora is a Maricultured colony. It looks in far better condition then when you first got it. It does look like it has browned up a little; maybe its the photo, but thats what it looks like to me.
I can tell you that I had Blue Millepora in my tank before, under my T5/LED light and the coral was completely blue from tip to base.
Generally, wild or maricultured pieces can take MONTHS - like 6 months or more to settle in and colour up. I did have a blue frag that was aquacultured. And another that was a maricultured frag. The aquacultured frag started to colour up within a few weeks, but the maricultred piece took more than 6 months to colour up.
Having said that, it really depends on the coral. I have a maricultured Lokani that is colouring at faster than any other piece.