Astair has a request that has something to do with the pokémon Clefairy.

The Request

After you add Clefairy to your Pokédex, check the blackboard in Professor Laventon's office to accept this request. Clefairy can be found in Coronet Highlands, near the Fabled Spring.

Talk to Astair

Go downstairs in Galaxy Hall and talk to Astair.

Observe Clefairy on a Full Moon

Go to Coronet Highlands. If it is not a full moon (watch for it to rise from the western sky at night), rest until nightfall and keep checking the moon. When it is full, go to the guidance marker and check on it.

Check the Sparkle

Before you go, check on the sparkle to get a Moon Stone.

Report Back to Astair

Go back to Jubilife Village, enter Galaxy Hall, go downstairs and talk to Astair.

Reward

You get 5 Salt Cake and 2 Exp. Candy M.