You need lime, i.e. quicklime (CaO), the stuff you use to make kalkwasser. To make kalkwasser you use about 2 teaspoons of lime powder per liter and let the white precipitate settle to the bottom and use only the clear fluid. But to make a super-strong solution I use 2 teaspoons lime in about a teacup of water. Mix it up and you get a white milky solution. I inject this straight into the Aptasia and if your aim is good he is a goner.Turn off your pumps and filters during the operation to avoid the slurry being flushed away from the target. Five or ten minutes later turn the pumps on again. You will get some white flaky stuff blowing around, it is just harmless slaked lime and other carbonates, it will soon disappear.
Be careful when you are mixing and using, lime is corrosive, which is why it kills the Aptasia. But the tank water neutralizes it, and you will get a miniscule rise in pH provided you don't treat more than dozen or so at a time.
Hope this helps. Feel free to ask if anything is unclear.
PS Interesting about your 6-line harassing your peppermint shrimp. I have a 6-line and it pays no attention to the skunk cleaner shrimps, but it is barely 2-inches long, maybe your is bigger and meaner