18h layover in Helsinki, can I leave the airport?

Both Finland (Helsinki) and Poland (Warsaw) are part of the Schengen zone. If your visa says "Schengen States" or the Polish equivalent, you can travel through any Schengen member state on the way to your destination and you will pass immigration the first time you cross a Schengen border (i.e. in Helsinki).

Your flight from Helsinki to Warsaw is effectively a domestic flight without immigration control. There will be an identity check which looks at your passport, but it won't be stamped again.