A few weeks ago I bought a "rescue" coral at an LFS. Great looking acan echinata that was receding. When I got it home noticed the white line at the edge where it was receding. Gave it an iodine dip, checked it out the next day, thought it might still be receding, so I took a picture of it, pulled it out and swabbed the edge with straight iodine. Next day compared the picture and even thought it was only a mm or two decided it was still receding, time to chop. I cut the edge off that had the white line, took off about 1/8" good flesh along with the bad. Gave it an iodine dip and hoped for the best. It worked, the coral is fine, healed up and growing. So, I'll recommend cutting if you can't get it to stop otherwise.
First pic, if you look at the far left side of the acan you can see the white line. Second is right after I cut it, third is 2-3 weeks later (a few minutes ago).