From the Super Mario Wiki
Octoboos are an undead ghost variant of Octoombas appearing in Super Mario Galaxy 2. They appear in the Haunty Halls Galaxy and Boo Moon Galaxy as regular enemies. In the first mission "A Glimmer of Bulb Berry", they are the first enemies to be encountered, and they can be found in groups while encircling a select area, behaving somewhat like Circling Boo Buddies.
Octoboos from Super Mario Galaxy 2
In the second mission "Sneaking Down the Creepy Corridor", Boo Mario has to avoid these enemies to progress through the area. Octoboos resemble white, legless Octoombas with ghostly tails. They primarily move around by floating in a set area, and they can also become transparent when the player uses a Spin on them or a tongue grab from Yoshi, thus avoiding any attack. Octoboos make small Octoomba sounds if Mario or Luigi is close to them. If Mario or Luigi spins close to one, it will disappear for one or two seconds and reappear again. While the Octoboo has disappeared, the characters can go through them. Unlike most members of the Octoomba family, they don't spit rocks.
Names in other languages
||Portmanteau of "Oktumba" (Octoomba) and "Buu Huu" (Boo)
||From "Polipoomba" (Octoomba) and Boo