A green slippery coating on hard surfaces. and sometimes on waterlogged lawns. Commonly occurs under the drip-line of trees but also seen on turf on heavy soil which has been overrolled, underfed and cut too closely. It can also be a problem on newly-seeded areas on badly drained soils.
On lawns - check the drainage is adequate, Spike/hollow tine compact soils, remove or prune overhanging trees and shrubs, routine treatment with lawn sand. Algae will return unless you can correct the basic cause.
This is currently no product on the market to control or stop algae forming within lawns.