Did you try power washing?

A 20% solution of muriatic acid and water with a nylon bristle brush and a garden hose.

I-
Thank you! And yes, we tried power washing but a good portion of the area is pooling when wet and now not draining due to the polymeric sand in the spaces where it might normally go. So, even though we try to sweep it off to the edges (new grass seeds and hay which are now also invading the space - delightful) that is not helping ...We will try acid a stone at a time. I have read that this can damage the stone , however? And what about the residual damage to new grass/plants?

I feel your pain. It happened to me too. I tried all summer to get rid of it. Most came off with power washing. The remainder disappeared after winter. Combination of snow and shovel? Lol

Thanks. Sigh...:( I guess better that this is late in summer than early.