Alolan Exeggutor in Pokemon GO can't actually be evolved from an Exeggcute. Exeggcutes will just evolve into the standard Kanto form of Exeggutor, but this doesn't mean that the Alolan Form is impossible to catch. Here's how you can add the Pokemon to your team.
Alolan Exeggutor: How to Catch
Alolan Exeggutor can only be found in the wild and in raid battles. Alolan Exeggutor is pretty rare in the wild, although starting on Sept. 22, along with Dodou, Pinsir, Wobuffet, Lapras, Snorlax, Furret, Mantine and Skarmory, will be appearing more often. Shiny Doduo and Dodrio will also be making their debut during this time. This event will last until Sept. 28.
Aside from the boosted spawns, Alolan Exeggutor can be found more often during sunny and windy weather, thanks to the weather boost from its Grass and Dragon typing. Alolan Exeggutor is also not just found in the wild, it can be found in raids as well.
Alolan Exeggutor can be found in Tier 3 raid battles so keep your eyes on local gyms, or even not so local gyms with the current deal on remote raid passes. If all else fails, using an incense can also help maximize your chances.