What is Mellobloom's trait?

The trait is called Phantom Opening, and it lets your whole party build Rose Marks, not just Mellobloom. Her own basic attack generates Rose Marks too, so you get stacks from her and from every teammate who lands a basic attack.

The marks pay out through critical hits. When a target has taken three critical hits, the rose mark blooms and heals all nearby allies for 5% of Mellobloom's HP. In practice, a team that basic attacks often and lands plenty of crits gets steady group healing without Mellobloom spending a skill on it.

That is why the trait fits the rest of her kit. Thorny Blessing makes the team's Grass critical hits spawn thorns beneath the enemy for extra damage, and her ultimate, Rose Garden, gives allies inside its circle a 70% increase to Grass critical damage for 30 seconds. Crits feed the bloom and the rest of her kit adds damage to those same crits, which is why she sits most naturally in a Grass team built around critical hits.