Looks like Sheep Sorrel (Rumex acetosella)

Here is a link that might be useful: Sheep Sorrel

Yes indeed, that is the one! Thank you! Any experience controlling it? All the information I'm seeing online seems to indicate it is rather difficult and that I should not attempt to pull it by hand, which is what I've been doing until now :-(

It has a very fine runner root that is hard to remove intact without breaking it....but you can get rid of it by pulling it up everytime you see it.....eventually you will starve the root by getting rid of the top....also ...never let it go to seed....you can spray it with a herbicide if you want to resort to chemicals.....

Sheep Sorrel has a root system that won't quit. The newest roots are thin and break easily while older ones can be pencil lead thick and pretty tough. Even the smallest bit of root can grow and produce more plants but constant removal of each and every plant and all of the roots you can will aid in keeping this under control.
Spraying with plant poisons may kill off the top growth but will not do much about the roots, so one would need to spray every week to keep these plants under control and that can get to be very expensive. I have found that removing the roots, carefully, keeps new plants under control for months.

Thank you for all the great advice! I read in a couple of places that Sheep Sorrel might also be an indication of high soil acidity, and perhaps applying lime, after a proper PH measurement, plus proper feeding, might help improve the soil in favor of grass and against Sheep Sorrel. Any thoughts about this?