#platax

Pinnate Batfish (Platax pinnatus), juvenile, family Ephippidae, order Acanthuriformes, Tulamben, Bali, Indonesia

The juveniles of this species mimic a toxic flatworm.

Pinnate Batfish (Platax pinnatus), juvenile, family Ephippidae, Lembeh Strait, Indonesia

It is believed that these juvenile fish, which look nothing like the adults, mimic poisonous flatworms.

Second photo, of a juvenile, from Wikipedia. First is mine of a sub-adult. 

A large spadefish (Fam. Ephippidae) native to warmer coastal areas in the Indo-West Pacific, and invasive in the Western Med. In Australia they’re found from the SW corner of WA, and all the way north and around to the south coast of New South Wales. 

Very young juvenile batfish resemble brown decaying leaves, but as the mature the fins become more and more vertically elongated, even more than in the photo at top. Juveniles usually school together under the watchful eye of a matriarch, but adults seem to be more solitary.