In New Pokémon Snap, each of the Lental region’s areas has its own huge Illumina Pokémon. Belusylva’s Illumina Pokémon is a Milotic who swims under the water, avoiding all your Illumina Orbs until you bring it to the surface.
On your first visit to the Belusylva Illumina Spot, you need to photograph Milotic while the Illumina effect is in action. You can activate it by throwing a couple of Illumina Orbs at it, but it will hide under the water out of reach.
To make Milotic come to the surface, hit it with a Fluffruit. After you hit it once, it’ll wince and surface, allowing you to throw Illumina Orbs at it freely. You’ll know when the Illumina effect has kicked in, as the Milotic will happily shout, and glowing swirls will appear on its body.
As it swims to different parts of the mount, the Illumina effect will wear off, and it’ll go back underwater. Throw more Fluffruit at it, and repeat the process to make it glow again.
Keeping the Illumina Pokémon glowing is important, as it’ll pose in new ways, use abilities, or even bring out new Pokémon to photograph.
On your second visit to the Belusylva Illumina Spot, you’ll see more Pokémon there, like Lotad and Ninetails.