Green Slime On Plastic Plants/ Cleaning Sponges

Are you able to take a pic of your plastic plants and the slime which is coming off them - is it possible that it's algae coating them?

Filter sponges should just be given a gentle squeeze in being-discarded tank water during the water change; that way, you'll not lose too much of the beneficial bacteria.

Only if they're disintegrating, which oughtn't to happen, should they be replaced - and, in that case, only replace 1/3 at a time to avoid losing too much beneficial bacteria at a time, then wait a few weeks before changing another 1/3, etc. The best way to only replace 1/3 is to cut any new filter sponge to the same size as the piece being removed.

Hope that helps.

Does this help identify the green slime?
http://www.theplantedtank.co.uk/algae.htm

Sounds like a filamentous algae. You could try a black out for a few days to try and kill it off. 3 days of the tank completely covered with no light getting in at all.