Sounds like STN. I would frag what you can as well.
There was an article of RC where there is a possibility of algae "growing into" the coral causing TN. If you do decide to frag, break the coral off it's base afterwards to see whether there's green inside the coral.
I've had a couple frags TN this way and both times there was green growing inside the coral. This can even happen with low PO4 rates.
I always make sure now when I attach frags that it is completely sealed in between the coral and the rubble. That way there is less chance where the algae can spread to inside the coral.
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