Commenting to follow the question. I've got something similar on my path, slipped on it a couple times. No clue what it is.

I know this sounds silly, but you haven't dropped any water-retaining granules, have you? I've done that in the past and it expands alarmingly in the rain.

Definitely not in my case. Not unless someone else has been working in the garden without me knowing. Good point though.

It is an algae not idea how you get rid of it though

Thanks moongrowe. If I come up with a removal solution I'll post it for you chezzarowney.