You are not alone rather than an answer! My asparagus is outside in raised beds and we, too, have just mulched with seaweed - the recent storms have brought up tonnes on the local shore. We mulched about 3 weeks ago now, and all I'd planned on doing was to cover the seaweed with our compost so it can rot down underneath rather than drying out too much on the surface - dry? here? SW Scotland? It's been windy so surface is dry and I'm thinking that the white 'mould' is more likely to be dried salts but the bitter cold NW wind is encouraging me to stay indoors and think seed sowing rather than investigate more thoroughly. There's always the tunnel to dig/manure though.