Thanks all for your input. I can't see any evidence of something haven eaten down to the "bone" and I cannot see eggs or anything else that could be associated with flatworms, so maybe it is not flatworms. I don't think it is normal growth either, none of these spots has ever taken off and started a new branch and some have been around long enough to do so. About the only other thing that I have not mentioned is that once the tip of a branch grew close to a nearby rock and normally it would have grown over it and encrusted it, but it never did. Instead, this same purple discoloration occurred just back from the tip and the branch never grew over the rock. It was as if something on the rock irritated it. That's what this looks like to me, sort of like a rash due to irritation but there is nothing obvious I can see doing the irritating. Mucous is never produced as it does when this species is perturbed. Also, I have seen normal color return to a spot that used to purple. I guess I will just be patient and keep an eye on it.
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