Well, Photon Geyser is essentially the best (or one of depending on opinion) Psychic move for Necrozma.
Photon Geyser is Necrozma's second highest psychic base-powered move, with the first being Prismatic Laser. However, with Prismatic Laser, the user will need to recharge on the second turn. Therefore, in two turns, you're actually comparing 200-base power vs. 160 base power - making Photon Geyser more useful in my opinion since it doesn't need a second turn to recharge.
Also, Photon Geyser actually uses either Necrozma's Atk or SpAtk stat, depending on which is higher. Therefore, you are assured to have the highest amount of power dealt out of Necrozma (without considering the opponent's Def or SpDef). Since Necrozma has a much better SpAtk stat, you may favour choosing this stat to be it's primary attacking stat, and therefore have a powerful Psychic special move. But, you may play around with Necrozma's stats and turn Photon Geyser into a powerful physical move. So, you can adjust Necrozma to suit your battle style with Photon Geyser.
In the end, you may choose Prismatic Laser > Photon Geyser depending if you like stronger attacks while sacrificing a turn - and which may turn up more useful in doubles, depending on your allies. But I think overall that Photon Geyser is a good move for Necrozma to have.
Your Choice! Hope I helped! :)
