He looks similar to Gary, except he has jellien eyes that are surrounded by orange flowers. He also has reddish-purple dots on his body and wears a brown flower pot instead of a shell.
Jellien Gary was created after the Jellien that SpongeBob brought home failed to eat the latter, so it ate Gary instead, while eating a plant at the same time. Unlike most of the other jelliens, Jellien Gary only appears for a few seconds and is never seen again after SpongeBob leaves his house.
Jellien Gary is the only Jellien Clone that is a mixture of two things - a plant and Gary.
Jellien Gary is never seen getting separated from the flower pot, so it's safe to assume that he doesn't revert back to just Gary, or it's possible that he did, off-screen.