Installed the Sweettank System last summer, it really helped with the odor problem.  I've recently noticed a green stain on the side of the boat coming from the vent, now there is also a brown stain.  What is it?  It isn't easily (mop & simple green) removed.

I am sure that Peg Hall will chime in with the correct answer but offhand I would think that you are going too long between pumpouts because your new system is keeping down that telltale odor that used to warn you that it was getting full. So when you hit a wake and rock the boat some of that dirty stuff is running up in the vent. If that is true you want to replace that vent line ASAP and set up a regular pumpout schedule whether it needs it or not.

The only thing I can think of that would cause a stain is liquid
flowing out the vent...so it sounds like you're allowing your tank to
get too full.  It's overflowing out the vent.