Your Arc Phone displays Muchlax's destination as Ramanas Island, and Mai asks you to investigate.

The Request

Talk to Mai at Heights Camp in the Obsidian Fieldlands to accept this request.

Go to Ramanas Island

Go southwest from Heights Camp to reach Ramanas Island. Approach the guidance marker and there will be a cutscene.

Report to Mai

Go back to Heights Camp and talk to Mai. There will be a cutscene.

Report to Cyllene

Go back to Jubilife Village and talk to Cyllene in Galaxy Hall.

Reward

Afterward, you get 30 Ultra Balls, 60 Apricorns, and 60 Tumblestones.