The Icy Merman is a Hardmodeenemy found in the Ice biome. Upon gaining line of sight to the player, it will stop in place and begin firing Icewater Spit projectiles similar to those fired by the Frost Staff, but with less frequency. Its projectiles also behave similarly to those of the Ice Elemental, but they are weaker in strength and fired more rapidly. The Icewater Spit attack inflicts the Chilleddebuff, and more rarely, Frozen.
As the Icy Merman stops in place when firing at the player, it is easily dispatched with a long-ranged weapon that fires more rapidly than its Icewater Spit attack, such as the Phoenix Blaster. This will easily lock it in place. Alternatively, if the player can use cover to get close to it without being hit by its projectile attack, it is vulnerable to melee weapons. However, if the player is caught by the Merman's projectile and it causes Chilled, it will be far more difficult to get into close range without being hit by more of their attacks.
A Nimbus Rod can be used to kill them effectively while they stand still.
The Bestiary entry for the Icy Merman: "Mermen rarely stray far from the ocean. Those that do can become lost, wander into cold caverns, and become icy undead fish men."