No danger
Powder snow on the wing is of no importance. The dangerous thing is rime ice on the leading edge (slats) or just behind it. By the time the air flow is to the flaps, any powder snow will quickly be blown off, as you see here. If the leading edge is disrupted, then the snow on the flaps will remain and not be dispersed, because the flow is already rendered turbulent and separated farther up the wing. Then there is no lift.