Sea Anemone

Sea Anemones, also described as giant octopuses , are sea creatures with long, stinging tentacles that appear in Yoshi's Story. They are only found in the level "Lots O'Fish". Sea Anemones are invincible and cannot be defeated. They act as stationary obstacles, moving their tentacles back and forth in an attempt to snare prey. Some Sea Anemones have the ability to move their arms in and out. They normally guard melons and coins, which Yoshi must snag with his tongue to avoid the tentacles.

Profiles and statistics
The big one is holding its tentacles in its den and waving them around.
 * Shogakukan guide: 大物は、デンとかまえて触手をゆらゆら.

Reference
Septimoni Sea Anemone