Other contributors have said "What do you do with the green liquid In astro knights?" is the same question as "Where is the green brick on astro knights?" If you believe that these are not asking the same thing and should be answered differently, click here
The green "fuel rod" in the saucer Excalibur is broken, and the only place to get a new one is the robot invader who is held in the dungeon. You can enter the back way through Mordred's underground...
The broken container of green liquid is called the "fuel rod" and powers the saucer. The only other device that might contain one if the captured robot in the dungeon.
(see solution at related...
if you have gotten to Mordred's lair here it is- go to the dungeon yada yada yada the alien in the cage will explode and you click on the owl and then click on the green thing in the cage that came...
The wall in Mordred's laboratory is dirt (push through it).
The wall of the dungeon has a gate (switch opens it from library entrance).
The mill has a secret dome opening device (release the gears...