Is this some kind of parasite crawling about in my yard?
It looks like a terrestrial ribbon worm.



https://upload.wikimedia.org/wikipedia/commons/a/ad/Terrestrial_Ribbon_Worm_%28Nemertini%29_%2822799575773%29.jpg

Nemertean worms are not leeches, but a different group of worms.  Most are marine but there are a few terrestrial ones.  This depicted worm is from Borneo and other terrestrial nemertean worms are from South Asia also.  But I have read that these can stow away in plants and are turning up in other parts of the world.  I found some small ones in San Diego that I thought were terrestrial leeches but were nemertean worms.

I would be very curious where you are in the world that big ribbon worms are partying in your yard.