The landlord said that "it's plastic stock, left from renovation on windows" from another apartment. It doesn't look like plastic to me, but then again I don't know much (very little actually) about construction materials. If it's asbestos can it be harmful just by passing by it while breathing in?
I would 99+% guarantee it is urethane foam . One of the raw material components is cyanide if you want to worry.
Not to worry.... it's not asbestos. Fiberboard.
That is rigid insulation it is not asbestos.