Will chlorine from a pool hurt a squirrel?

The raw chemical, yes - it is toxic and corrosive, but it's so unpleasant that the squirrel is unlikely to sample it. However the granular form does seem to emit a vapour or fine dust that could harm a creature that can't escape.

In water at swimming-pool dilution, probably not. The animal may well smell or taste it more sensitively than we could, so simply not drink enough to be harmed.