After you clear the massive mass outbreaks in the Crimson Mirelands, you go to the Cobalt Coastlands to investigate more.

The Request

After you complete Massive Mass Outbreak in the Mirelands, talk to Mai near the Front Gate of Jubilife Village. Say yes if you are ready to go to the Coastlands. There will be a cutscene. You will get some Sticky Globs.

Investigate

Talk to Mai if you want to feed Munchlax some Aguav Berries to reveal the massive mass outbreaks on the map. As before, catch or defeat all of the pokémon at a massive mass outbreak to clear it. Repeat this until you have cleared three of them. If you take too long and the massive mass outbreak ends, go back to Jubilife Village, then go to any area outside of the village, and keep going back and forth until another massive mass outbreak occurs.

Report to Mai

After you have cleared away all the pokémon in three massive mass outbreaks, talk to Mai.

Battle Akari or Rei

During a cutscene, Akari or Rei will challenge you to a battle. Go east from Beachside Camp in the Cobalt Coastlands to reach Ginkgo Landing. Talk to Akari or Rei, and agree to battle if you are ready. Akari or Rei has a level 60 Clefairy, a level 61 Staraptor, a level 62 Pikachu, a level 62 Prinplup, and a level 61 Mr. Mime.

Reward

You will get 30 Tumblestones, 30 Sky Tumblestones, and 30 Iron Chunks.