Have you fragged a shroom to see if it can be removed or popped to make sure it is not some sort of disease. That is very abnormal looking and I would have concerns.
You say you have had them for years but the growth is not that great for shrooms, even under cruddy lights, which you don't have??
Just a thought but maybe contact one of the coral gurus like Eric Borneman -etc.. and see what they think. You can also post on MD forum site if you cant reach him here. They have their own forums and can be a lot more specific as to what it might be. I do think that it is an abnormal growth and should be studied further other than guesses on a forum.
good luck