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