Hydro-Cryokinesis is the elemental ability to freeze water and all things that contain water.
With this power, merpeople can cool down and freeze water, from water molecules to human bodies. The best known mermaid who has this power is Emma Gilbert.
It can be combined with hydrokinesis to create a variety of things (i.e. ice balls, ice sculptures, etc.), but that would eventually melt or break. The hand movement for this power is a stiff "stop" sign, similar to the English sign language for the letter "B". This may symbolize that, when Emma is doing this, she's slowing down and stopping the water molecules.
If a mermaid with this power is in the Moon Pool during a special planetary alignment on a full moon, it could be enhanced to cryokinesis.