Carbon/charcoal in the filtration or not?

I use carbon only on an as needed basis, like after a major replanting and kicking up lots of gunk or after medication. I don't see a need to use it constantly as that can get expensive and is really unnecessary unless you're overstocked.

dark green stuff growing on the plants sounds like bearded black algae, a very tough algae problem to deal with. It's not caused by the use of carbon, most likely there is some nutrient/light/CO2 imbalance that is promoting it's growth. Trim the really affected leaves and bleach/scrub off the algae if it's on any equipment or decor. Dosing Flourish Excel may help, or adding CO2 another way. From there, have lots of plants and make sure they're growing well by adding the necessary nutrients and fertilizers.

http://www.thekrib.com/Plants/Algae/red-algae.html

Most likely you have an imbalance in lighting/ferts that is causing the algae. Carbon filters all ferts out of the water, so don't use it. It may be that the carbon was taking ferts out of the water, which would cause the algae. Or maybe you just are not dosing enough ferts. Try not using carbon for a while and see if that helps. If not, keep the carbon out and dose more ferts.

carbon has it's place . Using it can help reduce nasties in between water changes. many choose to use a scheduled water change, carbon is very effective at removing some meds from the water.

the dark green algae...is it slimey?

could be a few different algae.some are not true algae IE blue green algae is actually a cyanobacer(bacteria)

not................

I have C in my filter, but it's quite old. I stuffed the bag like a turkey 3 months ago, and I've let a little of it back out twice now to allow better water flow (C is no longer active but adds a little to mechanical aspect of filter, provides a little more surface area for bact. growth). I put fresh in ~1/year and I'd put fresh in following any course of meds.