Scalosian water was water from the planet Scalos which, when imbibed, hyper-accelerates the drinker so that they move so fast that they cannot be seen by anyone who is not so accelerated. The acceleration is caused by radioactive contamination which, with long term exposure, causes sterility in males.
Those newly accelerated are highly susceptible to cell damage. A simple scratch would be fatal, resulting in apparent rapid aging.
While under hyper-acceleration, phaser beams move slowly enough relative to the subject's perception of time that they can be easily avoided by simply stepping aside. (TOS: "Wink of an Eye")