Welcome to Canreef
Dottybacks, from my research, are notoriously aggressive, especially if they're the first ones in the tank. The only things I can suggest are removing the dottyback for a week or two (and keep the sixline in the tank), rearranging the rock, then re-introducing dottyback. Really don't know if that will reduce the dottyback's aggression, but it's the only thing I can think of.
BTW, how big is your tank?
HTH
