These things have been debunked aplenty in the past.
Basically, these machines are nothing but basic low-voltage electrolytic set-ups (AC-DC transformers attached to ferrous electrodes that corrode to generate rust when used to electrolyse the saline water in the footbath).
This scum is merely dissolved carbonates precipitating out of the water when the voltage is applied, and rust from the corrosion of a metal array that has to be periodically replaced. The "makeup" of the discolouration (what they claim to be "toxins") varies from machine to machine due to variations in the metals used for the electrodes.
The "scum" occurs whether a person sticks parts of their body in it or not.