Not an expert, but the documentation I have read on it describes it as the following:
It's ice that forms on the upper and/or lower surface of a wing/tailplane aft of the heated leading edge, either as a result of switching the heaters on after ice has previously formed on the leading edge or as a result of supercooled water droplets first freezing and then melting and running back towards the area aft of the leading edge and then refreezing.
Hope this answers your question.