White mark at rear of Dorsal fin where it meets body

[john10001]

One of my female Congo Tetra has a small white mark at the rear of her Dorsal fin where it meets the body. Are you able to help identify the issue and what I should use to treat it?

It was very difficult to photograph because they never keep still. They are only bold and confident at feeding time and timid and shy the rest of the time so I was fortunate to be able to get these few shots but it is still difficult to zoom in and see anything. I had to take it on my iPhone as my regular camera has the memory card full and I have been having issues downloading to my ancient computer.

[Gingerlove05]

It looks like a small scuff, possibly caused when they feed/chase each other.
Maybe a couple of extra small water changes to keep the water pristine and maybe a drop of melafix if you have any to help prevent secondary infections then just keep an eye on it make sure it doesn’t get any worse

[plankton]

Either that or a spot of fungus following a scuff perhaps.
do what @Gingerlove05 suggests and use an anti-fungal med if it doesn't clear up in a week, or gets bigger.