My cleaner shrimp does that too. Just like scaring them away, feeding the shrimp first then the zoas will only last so long. Once the shrimp is done with their portion, they're just going to scavenge for more. Or at least that's how my shrimp is.
I usually just hand feed my shrimp flakes, and occasionally tiny pieces would land on the zoas and paly's mouth and that would be their food. I dont bother spot feeding them anymore