Black band disease? Or something else?

[SmallFry]

Hi,

I've noticed on one of my acropora the encrusted base seemed to start dying off - initially I thought it was due to lack of light since the upper layer is starting to table out. Lately I'm not so sure, and the dieback seems to be continuing. I've looked at a bunch of pictures of black band disease, but found nothing exactly like this.


The darker area above the die off is not a sharp black band, but a gradual darkening from the normal pale yellow to a purplish brown.

Anyone seen this before?
Could this be lack of light underneath? I'm skeptical since I haven't seen this with any of my other corals..

Does cutting the coral off it's base above the brown area seem like a good plan? I'd rather not reduce it to tiny frags if I can avoid it, and I'd rather not panic and do it if this is something normal and I'm only starting to see it now my corals are getting bigger..

Thanks,
Rob.