What does Prismana Glacie's skill do?

Unconditional is the word doing the work. There is no element requirement, no health threshold, no combo to set up first. You use it and everyone hits harder. That makes Glacie a window opener: the 15 seconds is short enough that you want your heaviest hitter ready to spend it, and long enough to cover a full rotation from whoever is out front.

Because the buff lands on the whole team rather than on Glacie, it does not matter who is doing the damage when the clock runs. Glacie's own numbers are beside the point here. This is a support skill, and it is the reason to bring the Aniimo at all.

Glimmer Shot is a different skill

The Prismana form of Glacy also has Glimmer Shot, and that one is not a form bonus. It sits in Glacy's standard skill set, so any Glacy can bring it. Do not read it as the payoff for landing the Prismana version.

That distinction matters more than it sounds, because Prismana variants do not come with better stats, moves or abilities as a rule, and different forms of the same Aniimo share identical base stats. The exclusive skill is where the form actually changes what you can do.

How it compares with other Prismana exclusive skills

Most of the exclusive skills on other Prismana forms are damage, often with a typing that the base Aniimo does not have. Prismana Blazen has an exclusive Dark-type skill, though that form deals less break damage than other Break Aniimo. Prismana Fenmane is a Lightning Aniimo with an exclusive Light-element skill.

Flickering Shot is not built that way. It adds no new element to Glacie's kit and it does not care about matchups. Flat team damage is harder to misuse than an off-element attack, which is worth keeping in mind if you are deciding which Prismana to chase.

Not settled: How much of a Prismana form's strength comes from the form itself is unsettled. It is described both as a purely visual variant and as an empowered form that differs in more than appearance, so it is not yet clear whether Glacie brings anything beyond the exclusive skill and the altered model.