It looks much like the normal SpongeBob, except that it is made from a bubble and is completely transparent/pink. It has a shiny glow to it much like the other bubbles shown and appears to be the same height as SpongeBob.
It misses some of the specific details of SpongeBob, in particular his pants, his shirt, and his pores.
SpongeBob tries to replace Squidward and blows this bubble in order for him to make the scene of Squidward playing his clarinet. SpongeBob then yells at Bubble SpongeBob, while pretending to be Squidward, for interrupting his clarinet playing.