Start mixing the kalk paste... It's Aiptasia killing time!
They can multiple relatively quickly when they want to, so the sooner they are dealt with, the better. As long as you still don't have fish or inverts in the tank, I wouldn't worry about the pH or other water chemistry effects using generous amounts of kalk paste will have in a 6g tank. But, in the future, if you have some pop up periodically once you've started stocking the tank, I'd suggest being a a little more cautious with how much you use at a time.