From the Super Mario Wiki
Anenomes are sea creatures with long, stinging tentacles that appear in Yoshi's Story. They are only found in the level "Lots O'Fish!". Anenomes are invincible and cannot be defeated. They act as stationary obstacles, moving their tentacles back and forth in an attempt to snare prey. Some Anenomes have the ability to move their arms in and out. They normally guard Melons and Heart Coins, which Yoshi must snag with his tongue to avoid the tentacles.