I'm making a ninja turtle cake and they want to have lime green icing dripping down the side of the cake like slime. What is the best thing to do?

Piping gel colored icky green.

I would use piping gel..  I use the one made with gelatin, turns out great!  flavor and color it any way you like

Americolor Electric Green is a perfect slime color.  (I LOVE the Americolor electric gels.)

Ok. I thought about adding extra liquid to my icing to get it to be loose. Would that work? Haven't thought about piping gel though.

You can microwave 30 seconds per pound, pour in pastry bag, and pipe along edge with a #6 or #7 tip (depending on how thin your BC is) while helping it drizzle downward with heavy pressure where you want the drips to go.

Piping gel. I used it for blood running down a cake and it worked perfect.

Decogel is also perfect for slime!

a while ago I posted a video for edible ooze, it would go on top of your frosting as decoration. 

http://www.cakecentral.com/recipe/46016/edible-ninja-turtle-ooze