Do you have a photo? There are tons of different kinds of greenish growths you might have.
I agree with what Matt said, they will probably do better as a house plant than in a terrarium, and as a house plant you also have far less chance of unwanted growth.
Again, a photo would help...but if it looks more like the coloration of the soil surface is starting to turn greenish (it might even change the consistency of the surface soil to a little harder/more "brittle", rather than actually seeing a covering of fully-opaque growth), I'd guess it's just algae, which is actually completely harmless.
If it bothers you, though, and we confirm that it is just algae, neem oil should handle it. Take the plants out of the terrarium, spray it on the soil, and leave the plants outside the terrarium until it evaporates (in front of a fan on low for about an hour should suffice, unless it's a semi-warm day and you can put them outside for a bit). Green Light and Garden Safe are the best brands of neem oil.
Amazon has some you can order here: http://www.amazon.com/Green-Light-Organ ... 215&sr=8-6
Also Lowe's and Home Depot and probably Wal-Mart should sell Neem Oil (stuff that "contains" neem oil might hurt your plants, so only get it if it's plain old "neem oil").