It is best to clean your tools as soon as you have finished using them. It is much quicker and easier this way. The best way to clean your tools is to use warm water, a mild detergent (washing up liquid would be suitable) and a soft brush.
Simply wet, soap, brush and rinse.
Try to make sure that your feather edge is clean after use. Any concrete or plaster residue that dries on may crumble off when you next use it and mix with the newer materials, ruining your project.
Don’t worry though, it is not impossible to remove dried on debris; there are a couple of methods to try.
You can try gently rubbing off the dried on plaster or screed with either sandpaper or wire wool. You may also try using a dull chisel.
However Wonkee Donkee would recommend wire wool, as sandpaper and chisels can scratch the aluminium.