its gill curl.
in the early stage you can still fit it.
this is normally caused by two important factor.
poor water quality or limited tank space.
gill curl is most common on large fish such as red tail cat fish, arowana etc that are kept in tanks that are too small.
if you just notice the gill curl you can try do as much water change as possable. and if the size of tank is the issue then move it to a large aquarium.
given time it should improve.
however, if it doesn't help then you only have one more option.
you have to cut the gill that is curling and then add Mela fix and hope for the best, the gill willl grow back normal but this is the long and risky one as you might damage the gill or some freak accident might happen and damage the fish further.
good luck, keep us posted