mini jellies are of the slime family, weak to shadow, neutral to elemental and strong to pierching. Remember: damage number colors lie. You can check that if you shoot them with a shadow driver and a prisma driver (same base damage) and compare their damage output. Or any other elemental and shadow weapon combo that have the same base damage (Avenger/Faust, Blazebrand/Silent Nightblade)

Basically, they are of the same type as the Royal Jelly himself, being neutral to every damage type pre-nerf and same as the rest of the slimes post-nerf.