White Cottonball on tip of fin????? (2nd post)

[gailen]

We just got a small hippo tang from our lfs and it looks like his dorsal fin got damaged. The problem is now it is growing a white cottonball looking substance where the damage was. It this the healing process or an infection from the damage? We are fairly new to Marine and I know we should have quarantined but he is already in the tank and with the live rock almost impossible to catch without doing more damage. Any suggestions as to what this is and if it needs to be treated?
Thanks in advance
Posting again because first post looks like it linked to the wrong topic, not sure why.