From the Super Mario Wiki
Do-Drops are a species of waterdrop-like creatures in Yoshi's Story. Their names are a pun on dewdrop, a small drop of dew. The Do-Drops first appear in Stage 2-3: Jelly Pipe. They are usually found in narrow passageways, where they come out of a rusty and broken Pipe. They have a presumably sour taste, since a Yoshi will receive damage if they eat one. Because they are a liquid-based entity, eating one won't produce an Egg. Do-Drops are later found in Stage 5-3: Shy Guy Limbo, where they are dropped by clouds. They attack by bouncing off and colliding with any Yoshi in sight, dissolving themselves in the process.