Thanks everyone. I appreciate the feedback.
This growth has been happening with some of my gold hammers for about six months now. Aside from this, they are puffy, look healthy, and continue to produce new branching heads. Parameters have been stable with nitrates (20) and phosphates (.2) a little elevated, but stable for years.
Something I changed about eight months ago was my flow. I added a gyre. I also keep some of my hammers on a magnetic frag rack so they are higher up on the tank with higher flow and light. I have a lot of gold hammer frags, including some on the sandbed, but since I'm always moving them, I haven't noticed whether one or both have this issue. So the higher flow could be causing stress?
I also found a post on humblefish that attributed this issue to reproduction. I'm going to guess this is a stressed response to high flow, but don't know for sure. I've moved all hammers to my sandbed for now. we'll see how it goes.