Losing coolant out of radiator?

[diabloceto]

I have a 1972 Cheyenne with a 350 motor and I have been noticing lately that I am losing coolant out of the radiator. It is enough so when I look into the radiator I can see the top of the coils and such in it. I have not been seeing any water leaking out of it when I stop or any steam coming out anywhere when running the motor. The motor has not been running hot either. I did notice that the whole passenger side of the radiator running down the side is covered with a bunch of greenish junk. Any suggestions or is this normal? Thanks in advance for any help.