Montipora getting sick?

[xcross]

Hi,
I'm keeping a montipora I started from a small frag about a year ago. Its grown tremendously but in the last few months a large portion has turned black. When it first started, I thought it was just growing into its neighbor and getting stung. However, that is happening with the acropora on the right, and the montipora just bleaches on the margin and continues to grow around it. Can anyone identify what's going on with the black disease and offer advice?

Here's what it looked like in June:

By the end of August, you can see the diseased portion beginning along the margin of the lobe closest to its neighbor:

Here's what it looks like now. There's been tremendous growth of the lobes on the right but the affected lobe is nearly all black:

Here's a close up of the diseased portion:

Thanks
Chris

 

[TruReef]

Coral War.... someone has to lose...imo

 

[MammothReefer]

favia has big sweepers at nite