Exeggutor: 14-14-11 (86% IVs) with Zen headbutt/ Solar beam

wait mATE

Um, that's an awesome moveset (and the IVs are really good too, though IVs don't matter nearly as much as having the best moves).

What level are you, and what CP is the Exeggutor? The only reason I can think you wouldn't want to power it up is if it has a CP value of like 500 or something similarly low.

You didn't give any information about your trainer level, CP or level of the Pokémon, how much stardust do you have etc.. Impossible to answer the question without at least this much data.

why not if 14 is ATT, with okay iv, good attacking move set, once u reach high level, your star dust will be back

If you want a great Exeggutor for attacking then wait no more and power up this beast. If you prefer to use one for defending gyms then I would probably look for one with another moveset.

I'm level 27 and my exeggutor is at 87% maxed at 2351, max possible is 2406. It'd be nice to have a perfect one, but is 50 cp really worth hatching an even better one than you have now, on top of having to get the right moveset with him? Power him up.

That moveset is best for attacking gyms so I would not power it up as you can use it to fight larger water types and get max prestige. If your gonna power up mons go for ones you are gonna leave in gyms. With a Exeggutor you want confusion and psy/seed/solar. That way you get back the stardust you put in and the extra gold is gravy.
I try to have a counter for water pokemon at every 300 cp interval. So a victreebell/vileplume/exeggutor /venus/tangela/parasect starting at 600 cp and a mon at every 300 cp over that. ie 600cp/900cp/1200cp/ 1500cp/1800cp/2100cp. Tbh since water pokemon are so prevalent I actually have counters at every 100cp if not more. That way you can quickly and efficiently max prestige any gym you come across. I try to have the same counters at 200-300 cp intervals for fire and grass as well. I rarely power up these counters, i just estimate the evolved cp and evolve the mons that have the CP i need and hope for the right moveset.

Since you already made the right decision, I will add that my TreeHead with the same IV and moveset wrecks things. I'll even toss him in a gym as a defender from time to time just cuz his CP gets so high. This shouldn't even be a question!