After you return from the Ultra Wormhole, you can continue your Island Challenge with Captain Mina's trial.
Mina's house is in Seafolk Village, in the Whiscash-shaped boat. Talk to Mina when you are ready for the trial.
First, you have to battle Mina. She has a level 51 Mawile, a level 51 Granbull, and a level 51 Ribombee. You get 6120 Pokédollars for winning.
After the battle, Mina gives you the Pink Petal. She says that to complete her trial, you have to visit all the other island Captains and get a petal from them as well.
Captain Ilima is in Hau'oli Cemetery. When you go there, Hau gives you a Max Elixir. You will find Ilima in front of one of the graves on the right. Talk to him and you can battle him if you are ready. Captain Ilima has a level 51 Gumshoos, a level 51 Smeargle, and a level 51 Komala. You get 6120 Pokédollars for winning. After the battle, Ilima gives you the Orange Petal. He can take you to the next captain if you wish.
Go into Lush Jungle. After a cutscene, if you are playing Pokémon Ultra Sun, you will battle Captain Mallow, who has a level 51 Trevenant, a level 51 Tsareena, and a level 51 Shiinotic. If you are playing Pokémon Ultra Moon, you will battle Captain Lana, who has a level 51 Lanturn, a level 51 Cloyster, and a level 51 Araquanid. You get 6120 Pokédollars for winning. After the battle, Captain Mallow gives you the Green Petal, and Captain Lana gives you the Blue Petal.
Captain Kiawe is in Wela Volcano Park. You can talk to the Trial Guide to the right of the barricade to heal up your pokémon. Go to the top of the mountain and talk to the hiker. After a cutscene you can battle Kiawe. Captain Kiawe has a level 51 Arcanine, a level 51 Talonflame, and a level 51 Alolan Marowak. You get 6120 Pokédollars for winning.
After the battle, you have to battle Hiker David. He has a level 48 Magmar. You get 1536 Pokédollars for winning.
Afterward, Kiawe gives you the Red Petal.
Sophocles is in Hokulani Observatory. Go to the back hallway of the observatory and go through the middle door to find Sophocles. Talk to him to battle. Captain Sophocles has a level 51 Togedemaru, a level 51 Magnezone, and a level 51 Alolan Golem. You get 6120 Pokédollars for winning. Afterward, Sophocles gives you the Yellow Petal.
Acerola is in Aether House in Route 15. When you try to go up the stairs, there will be a cutscene, and you get a Grip Claw.
Afterward, go into Aether House. Inside, Nanu gives you the Purple Petal.
After you have all seven petals, go back to Seafolk Village, go into the Whiscash-shaped boat on the east side of the village, and talk to Mina. She gives you the Rainbow Flower.
After that, you must battle the level 55 Totem Ribombee. It is holding an Occa Berry, which reduces the damage from a supereffective Fire-type attack. It can call ally pokémon into battle, such as Pelipper and Blissey.
Afterward, Mina gives you Fairium Z.
When you go outside, Hapu tells you that she will be waiting for you on Exeggutor Island. Go to the southwest corner of the village and talk to the chief to take a boat to Exeggutor Island.
Kahuna Hapu's Grand Trial
Go north and talk to the northernmost Exeggutor to go up to the upper part of the island. Then go north and approach Hapu. She will challenge you to a battle. If you are not ready, you can say that you need to prepare.
Kahuna Hapu has a level 53 Golurk, a level 53 Flygon, a level 54 Mudsdale, and a level 53 Gastrodon. You get 8640 Pokédollars for winning.
After the battle, Hapu gives you Groundium Z.
Your next destination is Mount Lanakila, which is north of Tapu Village on Ula'ula Island.