Clean your screeding level as you go along and as soon as you have finished using it. It is much quicker and easier this way. The best way to clean it is to use warm water, a mild detergent (washing up liquid would be suitable) and a soft(ish) brush.
Simply wet, soap, brush and rinse.
Any concrete or plaster residue that dries on may crumble off when you next use the tool and mix with the newer materials, ruining your project. Don’t worry though – it’s 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. For tougher materials, you may also try gently using a dull chisel.