Is this invasive weed (no roots and spreading in my lawn wherever grass is sparse) an algae and how do I treat it on the lawn?

I'm having trouble deciding exactly what this is - it almost looks like some kind of lichen - is your soil very damp and is the lawn growing in shade?
If it is a lichen of some kind, then conditions in your lawn need improving - scarify in September, spike the lawn all over by inserting a garden fork to the depth of the tines and removing it to leave small holes, then brush in lawn sand. Re seed the bare areas. Next year, mow regularly and use a proprietary lawn weed feed and mosskiller treatment every six weeks from April through to July/August.

Thank you for the reply....... 95% of my lawn is A1 = zero weeds, nice thick/short grass - cut every second day (obsessive my wife says!)

Like many areas, Northumberland this year has been very wet & humid...

I suspect that you are right and this is a lichen... on the lawn its not confined to damp areas, but to reseeded and newly weeded areas.
Its not confined to shade either!

Its doing awfully well!!!!

Is the bottom line that there is no killer - just scrape it off wherever it occurs and then prepare that area in best time honoured fashion for reseeding (better this time)...?

and that scarifying at the end of Sept would be a good idea too?

Many thanks (again) Martin

Moss killer treatments might work on it, but I've no experience of lichen growing in lawns. I would scarify and aerate as suggested, and reseed the bare areas after scraping off, as you say. You might be better off leaving it at least a week between cuts too - unless its a bowling green or cricket table, it doesn't need to be cut that often and will be healthier with less, as its lawn grass.

I have this, I think its one of the liverworts
Mine was on the lawn but at the edges and only under tall shrubs such as Weigelia and Buddlia. I have removed the grass and made the area under these shrubs grass free and that did it for me.
I assume its damp and dark grassy conditions mine have grown in as I have none anywhere else. Under tress the grass tends to be dryer and I have no problems there. RHS have advice on this type of problem. Good luck its really ugly.

Yup liverwort not lichen - seems that Armillatox used at the same rate you would use for killing moss on your lawn will do the deed.

Thanks moon grower if I get anymore I will try armillatox as other weed killers didnt work for me.

Yes it is liverwort as Moongrower and Denise say. Propogated when raindrops go in the cups on the leaves. The spores are splashed out.

That makes sence to me Doctorbob1

Thank you everyone... what an amazingly helpful site! I'll get to work tomorrow!
regards Martin

Go Martin!

It is Liverwort. Your lawn is too wet/damp and needs airating and top dressing with sand.