From the Super Mario Wiki
Bubble Dayzees are purple Crazee Dayzees that will walk a short distance and then blow out bubbles, similar to Barney Bubble. They are in three levels of all the games with Yoshi's Island as the title with the exception of Yoshi's Island DS.
Yoshi can eat the bubbles and then spit them back out to destroy almost any enemy. He can also bounce off of them to reach greater heights, but the bubbles he touches will pop in the process. Once Yoshi blows them out he'll have to lick up another to keep the weapon. The bubbles can only run out if they all pop and the Bubble Dayzee is gone.