I wonder if you could have a look at these pictures of a strange growth on the lawn of our neighbours. It starts off as a yellow patch looking like slimy mucus, then it turns white and seems to swell up to a foamy consistency, it turns black at the next stage. It does not smell of anything and the grass underneath seems not affected so far. There are lots of patches variying in size from 10cm diameter to about 20cm. It looks awfull and we are worried it might be something nasty so we don’t want it to spread all over.
There are many kinds and they grow on rotting material, including dead grass, usually in damp weather and warmish times, late summer and autumn. Most of the time they exist as single cell microscopic organisms but can aggregate in the right conditions.
These appear and disappear and do no harm but can be very slippery. The change of colour has to to with the production of spore masses.