What type is Prismana Melabloom?

The ordinary Melabloom is Grass only, so the Light element is what its Prismana form adds on top. You will also see the element called holy in the game, and the name spelled Mellobloom, Mellow Bloom or Melloblum. They are all the same creature and the same type.

That second element matters in fights. Attacking with a type advantage deals 1.6 times normal damage, and Light is one of the elements the in-game type chart lists strengths against. Prismana Turbo is weak to Light, which makes it the obvious target for a Light-typed attacker.

Adding an element is how Prismana forms tend to work. Turbo's Prismana form does the same thing, adding Dark to its base Wind for Dark and Wind dual typing. Glacy is the exception that is not pinned down: its forms are listed as Water and Ice, and its base form is also given as Water and Holy.

Not settled: Prismana Melabloom's type is settled, but its stats are not. Prismana forms are described both as having higher base stats than standard forms and as having identical stats, which would make them purely cosmetic. The extra Light type does not tell you which of those is true.