A different type of De-icing
 

Makes sense but it was the first time I had ever seen it. Temperature about 3 degrees C and a bit of drizzle. Flight had arrived from a trans-Atlantic crossing and was departing for a short 40 minute flight.

De-iced on the bottom of the wing only because the ice frozen contamination was more than 3 mm thick. Took longer than normal due to engines having to be shut down at the de-icing bay.

Probably ridging icicles due to a cold soaked wing and drizzle running under the wing.

Used to see it early mornings in winter on an A320, if the previous flight landed with 6t or more of fuel, it was cold soaked and the hoar frost would form on the underside of the wing.

Pump in some warm fuel and it would sort itself out.