I'm not gonna argue if it's anchor worm or not......few years back when I had some anchor worm introduced into one of my tanks from some feeders.....the initial stage was just a small worm attached to the edge of the fish scale, and eventually gets under the scale, and the area gets red.....as the worm grows......
In this case, yes I see the little tip that looks like a worm, and what looks like a cotton ball....this is exactly the same as I had.....exactly in the same area of the fish (tailfin, and tail stub)....the cotton ball does not get any bigger, nor does the worm like thing.....it just spreads around more.....and eventually you'll end up with a bunch.....I had a bit more....as I only caught it much later, as I don't use the light on the aquarium much....
I treated with salt....to no avail.....the aquarium was a 150 gallon....I did 4 treatments with clout, 25 capsules at at time....again to no avail......
Only with the previous mention of treatment did it go away, after 2 bottles, and two treatments....
cheers