From the Super Mario Wiki
Ick Worms are enemies from Yoshi's Story. They are only found in one level, stage 4-4: Neuron Jungle. Ick Worms are large blue caterpillar-like insects that float down from the treetops in the Neuron Jungle. If a Yoshi eats one, he will get an egg, but at the same time, lose a flower petal. Only Blue Yoshi can eat them without getting damaged. However, the leaves that Ick Worms soar on will heal all of a Yoshi's health if eaten.