[Bones]

One thing I can suggest is if you can do it (Or haven't already), install a drain valve to the system as I've done for maintenance purposes.

I have one in the lowest part of the system's tubing and that makes it a snap to drain the coolant out.
As for the green stuff, you can use a drop or two of bleach if you want but be careful since bleach is corrosive to metals. I've used it that way with mine and so far, no issues with pitting or the like. All it takes is a tiny amount to work and also be sure to change the water on a regular basis - The drain valve is very useful for that too.
You can also find a biocide that works such as what's used in aquariums to control issues with algae and other problems like that. Be sure whatever you do use with those won't affect the metal parts of your system.
Use sparingly and it should be OK.