Well, this is the tricky part. You need to get the lemon juice INTO the aiptasia. Best bet is use a needle and get really close to it. Like, so close it closes up and hides in the rock. Then stand there perfectly still. When it pops back out, squirt and jab in the same motion.
If you can get close enough without scaring it, even better. Saves time. Kalk
paste is about the only thing you wouldn't need to get into it.