from "Apparent mimicry of jellyfish by juvenile pomfret, Pampus chinensis (Teleostei: Stromateidae)". H. H. Tan. Pp. 139-142. [PDF, 431 KB]
One juvenile was observed swimming in an erratic manner somewhat like a jellyfish!
There's a video clip of a swimming juvenile pomfret (24.5 MB)
In colouration, juvenile fish also resembles a jellyfish.
Photo from the article.In addition, when it was caught, the fish produced "copious quantities of clear mucus".
The article also shares other examples of juvenile fishes mimicking poisonous animals.