Can't tell from the videos which are not that good for observing up close. And ich uses a lot of the body, not just the tail.
What would help would be:
1) good up close pictures of the whole body
2) some history of all the fish in the tank including those that were in the tank in the last month
3) some description of swimming behavior
4) some description of medical history: did the tank have ich before? How long have you observed that mark?
5) temperature of the tank, current ammonia, nitrite, nitrate and pH level.
Without most of that, it's pure guess work as to what it is and what should be done. Also, increasing the temps without knowing the ammonia levels and what that mark is could be a bad choice.