White spot on slabs?

[Holmbrae]

Garden slabs have developed a mass of white spots that look like bird droppings but are not, any ideas

[brenda]

Possibly a kind of lichen.

[Jacko439]

Have the same problem, I to think that it's lichen, It's all over my tarmacadam drive and over the pavements too. Not too serious a problem though, will come off with a pressure washer or scrubbing.