of course you can get algea in a closed loop setup. at least for a little bit until it consumes all the oxygen in the water.. and then you get the dead material of bacteria and plant life. It's fun stuff. The best way to combat this is to never turn the pump off....ever. But eventually if you dont have the water mixed with something toxic, things will grow in it or metal will oxidize if mixed incorrectly. If water sits anywhere in your system for some length of time there can be a buildup of stuff..
If you do get algea (or corrosion) in a system that has been up for a while with no sign of such algea or corrosion within the first week of use then there is likely an air leak in the system. If it was contaminated at the time of filling you would have seen things probably after the 3-4th day... of course it may take a bit longer if your water is in constant motion during this time.