From the Super Mario Wiki
A Calm Fishing Boo (or C. Fishing Boo) is an enemy found on Vibe Island in Super Princess Peach.
Like other Calm enemies, Calm Fishing Boos are usually found asleep. If Princess Peach moves too fast, she might wake it up. When it does so, it will attack like a regular Fishin' Boo.
According to the game's Glossary, one should "Walk by quietly when it's sleeping."