The term as it pertains to human behavior originates from the video game Halo. Such a player will hide, waiting for the right opportunity to come out and get a kill, only to retreat into hiding again. This player will often hide to the detriment of their team, staying in a safe area while their team mates are killed. This kind of play is selfish an can be detrimental to their team.
The term can also be directly realted to poker, more specifally Texas Hold'em. One who makes irrational decisions, playing weak starting hands, even when facing a large raise. Such an idividual is also easily intimated at the poker table, folding strong hands to others who have coaxed them into believing their hand is stronger. An example of a move that clown fish might make is calling and all-in bet with a 2-3 off suit, one of the weakest starting hands in Texas Hold'em.
Stop hiding like such a clown fish and help me shoot these guys! (Halo)
That was a clown fish move, calling an all in with a 2-3 off suit.
Named after the clownfish's ability to change its sex.