Is the white or pink buildup in my cat’s fountain harmful?

The white crust is mineral scale from hard water and is not toxic, but it is not something to leave alone either. A pink or reddish film is usually a biofilm of bacteria and yeast that thrives on the wet plastic, and that can genuinely irritate a cat’s chin (a version of the same reaction some cats get to plastic bowls). Clean the fountain fully, not just a rinse, at least once a week, more often in a hard-water area, and replace the filter on the schedule the manufacturer lists rather than by eye.