i've encounter this once when i put too much rabbit manure into the water. vacuum or throw away as much manure in the bottom as possible & dump all the water. start with fresh one.
I'm pretty sure it's protein foam (the same foam you see at the ocean beaches after the waves come and go); it's simply a byproduct of the fish in the system; the green color is algae that's feeding on it. You can net it out with a fine net swirled in a figure of eight fashion across the water surface, rinse it with a garden hose and return the duckweed to the tank. You may be feeding the fish too heavily for the current water temp, or simply need to increase the water turnover rate per hour through your system. THERE'S NO NEED TO DUMP ALL YOUR WATER!
Why are people putting rabbit manure into a system? Seems like a great way to increase contamination issues.