Disappearing Boo Buddy
From the Super Mario Wiki
A Disappearing Boo Buddy is a type of Boo found only in Super Mario World. They are only found in the Sunken Ghost Ship and cannot be defeated. They are found in a group randomly scattered throughout the screen and are completely motionless until they simultaneously vanish and reappear elsewhere. The best way to deal with them is to move very slowly to avoid colliding into one. They are similar to the Boo Buddy Swarm found in the Donut Ghost House and Forest Ghost House, but Boo Buddy Swarms turn transparent rather than disappearing, move rather than staying still, and are a tighter mass.