I've been going through this myself for the last 4 months or so. I've had health heads all of a sudden start to up root from the stalk and over 2 - 3 days fall off. The fallen head would actually be fine, well at least the ones that didn't fall into the rocks. I would lose 1 or 2 heads from a colony but the remaining colony would be fine, as would other colonies of frogspawn and hammers. Everything would then by fine and then all of a sudden a couple more heads would fall off. This was happening with all different colonies, some near colonies where it happen before others far away.
What also was happening was that the babies, that were growing under the head, start to get bigger and bigger. Some are actually almost full size now.
During this period my levels have been very good, nitrates went up a little which is most likely from some of the head dying, but all the other corals, inverts, fish and clams were fine.
There really isn't much I could find about this outside of it possibly being a-sexually reproduction. Either way I've lost a lot of heads but have even more babies growing in their place.
