this is quite a controversial issue that has been been discussed here several times. personally, i often continue a session with a good puerh or oolong another day, or steep it overnight. i've never had any issue with leaving the leaves in a closed ceramic teapot overnight on the table (never more than 24 hrs!!!). this of course depends on your location (i'm in canada) - on the other hand, most people in southeast asia, where the climate is more warm and humid and mold develops faster, would probably strongly advise against it (see http://teaism99.com/2014/02/06/the-one/
). anyways, with green tea it might not be worth.