The water is terraformer'd The metal is held at bay with all manner of shielding, and additionally vaporized with plasma.
The Metal used Heat Resistance and Fusion.
It is highly effective.
Chunks of metal rain from the sky and make the liquid metal grow in size. While water seems to be being generated from inside the city now...
.... Fusion? It is reflected by shielding, absorbing the supermetal rather than helping it absorb. Each targeted Matoran mind threatens and conquer individual, orphaned drops and fragments of Stalker's mind and body, gaining his powers in the process. Thx Terraformers wage war on the Water. Weakness/jutlin beams wage war on the metal.