That green cap is your access to the coolant system, the bottle also contains a float on the inside that often sticks and causes the light to come on in the DIM (drivers information module). One other source is the connection for the float sensor, sometimes the pins on the connector are not connected correctly,one maybe backed out and not making good connection. Take the cap off your bottle and with a small screw driver see if you can push the float up and down and get the light to go out, if not you may need to have the bottle replaced.