It's Snow-In-Summer (Cerastium tomentosum). I planted it from seed years ago to bloom with the poppies. The plan has been marginally successful. Most years the poppies are finished before it's in full color.
Just like with all the ground covers I have , this one is mass planted in the rockwall garden and it was chosen (as all the others were) because it looks good even when it's not in flower. Good foliage is very important because they only flower for about two weeks.
It is not in full bloom yet, but when it is, it will look pure white like a big mound of snow.
At about the same time the white blooms, this yellow sedum (my 4th ground cover planting) will also be in full flower.
My plan is to eventually have the entire rockwall garden in ground covers, shrubs and just a few flowers. I am planning for my golden years. :)
Two years ago I added two of these Royal Purple Smoke trees (Cotinus coggygria 'Royal Purple'). They are just tiny now, but can get up to 10 feet tall in 10 years. Not too long ago Lostroses posted a picture of a huge one in it's full smokey glory.
It's looks like this one may be ready to take up smoking this year.