Electrolysis rust removal - side products - identify lead
Add the information regarding the composition regarding the electrodes. However lead is very heavy it would not like to float on the top. Secondly, lead compounds are mostly white with an exception of oxides (yellow/ red) and iodide (yellowish). Most likely the black floating scum is carbon in iron. Wear gloves, and wash your object with a mild abrasive such as a baking soda, and it should be fine. After all it was meant as a hand tool. It will not become "toxic" after a brief electrolysis.