What is the white layer on the compost around my pot plants?

Angle shades moth Anthracnose Box blight Box tree caterpillar Canker — Bacterial Canker — Bleeding Canker — Fungal Capsids Centipedes Chafer grubs Coral spot Cutworms Earthworms Earwigs Eelworms (leaf & stem) Eelworms (soil-borne) Froghopper Fuchsia gall mite Garden Ants Grey mould Honey fungus Leaf cutter bees Leaf Rolling Caterpillars Leaf spots — Fungal Leatherjacket Lily beetle Mealy bugs Mildew — Downy Mildew — Powdery Millipedes Pear midge Phytophthora root rot Rose Black spot Rusts Sawfly — Berberis Sawfly — Gooseberry Scale insects Sciarid flies Slugs Snails Sooty moulds Thrips Vine weevil Whitefly Wireworms Woodlice Woolly aphid