Greenish-black slippery slime forms in patches, usually on areas of poor, bare grass Cause Algae. It is mainly found on lawns with poor drainage and poor aeration. Areas under trees are also affected because of the rain drips from the canopy makes matters worse.
In itself the algae is unsightly but isn't too damaging. However, it does point to the poor growing conditions of the lawn as a whole.
Removing the algae from the grass is relatively easy. Using lawn sand regularly will help stop it reappearing. However, unless the growing conditions are improved the algae will return in time. This can be achieved by aerating the lawn with an aearator or fork, scarify it to remove the moss build up and top-dress.