Im thinking it might be the white edges around a fresh bite out of fins if you look really close. Might just be a my fish thing, I'm not a pro by any means. I know it doesn't look like ick but it does make sense with the agression...
But I'm thinking he will go for one of those week long water changes.
If you do have a camera of sorts you can try a few different things... going from on the floor out of direct view of them, zooming in, sitting still, darken the room your in so they cant see you as well, or vids work wonderful too. They have to stop sometime it just takes a bit.