I was just checking the water level in the overflow tank, it was low so I Thought I'd give it a little water. The water was already green, natrually, and as remove the cap/hose, I found something slimy, more like creamy, light green substance on the tip of the hose. It was not there when I last refilled, 2 months ago.

What could that be?

 

just buildup maybe? if your worried...maybe do a coolant flush?