Dunno, never left them out of the packet as I always reseal it... Well, I say 'always' but I've only opened it once and now they're all in a box somewhere gathering dust.
However, when I was younger I used to have goo like this and it didn't go hard but went liquidy, bits dissolved and the rest evaporated over time. Then again, that was from a completely different line and company so you never know.
