I don't think so. It may come right off without extreme measures. Just a matter of getting it wet through and through with surfactant as the gray that is surely underneith will give it all up rather easy. Some butyl or tsp along with percarb should do. Covering them plants might be sound advice though..
This was just with garden hose in a somewhat controled acidified bleach experiment. Not nearly as thick as what you got Matt but it just goes to show how weak the stuff can be to get off:
Although not same gross buildup this deck did fine with boosted NaOH and didn't kill any flowers or plants that were overgrown all over the railngs.The first pic is the test section with garden hose and I ended up diluting about half during actual job. The stair rails looked same as Ricks pic:
