I believe these to be bacterial in nature.
I have seen it happen on Trachs, Lobos, euphylia, bubbles, Blasto Wellsi(but not on merletti), and Acans. They tend to either bleach or recede. The bleaching I believe is caused by too much light or the wrong color of light. Then once weekend, they recede and die.
That is why I do the hydrogen peroxide baths in SW. These baths seem to stop the recession dead in its tracks. It may take treating everyday or every other day for a week to a week in a half. then things seem to look better.
I always recommend a full barage of water changes and to change all chemical media. Just in case it is a concentration of heavy metals or something that we are not testing for.
I have been experimineting lately with this product, but still prefer the hydrogen peroxide baths....
http://www.twolittlefishies.com/documents/1187368847.pdf