Those are mesentery filaments inside your anemone. they are normal structure, neither good nor bad. Your anemone looks a little stressed, likely due to be in in a new tank (new for him I would guess). Your tank has coralline algae, so I don't think it is a new set up.
Very nice looking anemone! Do you know what kind of BTA it is? I agree with above, not good or bad. Ideally they will be less visible after it settles in.