Acid

Acid pools, geysers, etc. provide an environmental hazard. Realistically these should appear largely indistinguishable from normal watery ones until you get exposed to them, but in fiction they often share the standard color of poison though possibly much lighter variety (light or lime green).

Technically this also can work as a form of weapon or attack, but in games it usually occurs in identical manner to poison and has actually no properties of acid.