(I am playing Ultra Sun and have just captured him)
I remember seeing some answers on Necrozma's moveset page that show him learning both Photon Geyser and Solgaleo/Lunala's signature moves depending on it's form. I see that Necrozma learns Photon Geyser at lv. 50, but am wondering how he obtains Sunteel Strike or Moongeist Beam. Is there a certain move he needs to have in his moveset (such as Kyurem learning Glaciate or Scary Face)? Or was I misinformed? Thanks.
Because Necrozma fuses with Solgaleo and Lunala, it acquires their signature moves. No, it does not need any move to acquire either Sunsteel Strike or Moongeist Beam, just needs to learn them.
To fuse Necrozma, you must use the N-Solarizer or N-Lunarizer.