That me dear is the dreaded Liverwort. It just lurves the conditions of seed trays and house plants. Not easy to kill off either. If you can find an algicide which does not harm plants then use it. Otherwise you need to scrape it all off your house plant soil and replenish with fresh. Sadly on seed trays it is a bit harder as anything you do will damage the seeds underneath.
One bit of advice I was given was only to water inside plants and seed trays with tap water, rather than rain water, not tried it to see if it works though.

That was quick Owdboggy and thankyou for help, advice taken on board.

if i find it on my pots i take it off and bin it, then renew acouple of top inches of compost in the pot

And not in compost bin but wheelie bin

The spores are as ubiquitous as moss or fern ones!