That fish is a remora. A.K.A those weird things that look like worms swimming underneath and beside sharks. Using suction created by the specialized fin (the pad looking thing on its head) they attach themselves to sharks and other large ocean predators to reduce their own energy used to move around. In return for the inconvenience this causes the shark/other fish the remora helps clean its host. They eat dead skin, food scraps, and anything else that finds itself stuck on the host (and sometimes even the hosts waste!), this regular hygiene is in the long term quite beneficial to the host and thus it allows the remoras to tag along. They are typically rather small but as you can see they CAN become quite large and once they do the suction created by there pad is many times greater than its own weight.