My problem is that after the third bule wash I find a greasy,white scum forms on the top of the bio-diesel.This tests very caustic although the wash water shows neutral.This also forms on the sides of the processor and is difficulto shift.All the initial phases of the process are very carefully done and all is well until this final stage. Farm

Without photos, and or more info, it is hard to be sure, but that sounds like soap scum.

It is most likelt soap bubbles. I find that as the methanol is removed with early washes some soap which is in the water forms bubble from the airation.

Just skim these off with a fish net or a piece of panty hose on a coat hanger.

You will be fine and it will settle as it dries.