Aquatic Dash◆◆ The merfolk warrior swims and attacks in one of two patterns. They either Swim twice and Strike at the end of their movement, or Swim once and Strike at any point during their movement.
Source: Pathfinder Monster Core
Running this fight? Add the merfolk warrior to the initiative tracker: type its name and the AC, hit points and Perception for initiative fill in, and the book button shows this stat block mid-fight.