Yowch.
The first thing I'd do is a closer inspection to make sure it's not brown jelly causing it to recede.
I've had a couple battles with brown jelly so far in my tank... I lost my first frogspawn to it, and a good portion of a hammer colony. It also killed a couple of heads on another hammer.
It's nasty stuff, but a couple dips in an iodine solution (like seachem's reef dip) combined with 'painting' the infected area with iodine during the dip clears it up pretty quickly.
Unfortunately, the infected head usually dies (at least it did in my case) but it keeps the brown jelly protozoans from spreading to neighboring heads.
I hope that's not the case, but I can't see too clearly in the photo what's happening to it to give you a definitive answer.
Crossing my fingers for ya.
Edit- It also could just be propagating itself, but I'm not sure if frogspawn do it this way.