Bluestone stain woes - help! Pics attached

We have bluestone around our pool and as part of a patio in the back of our house. We have a large maple tree. We came home from being away for a few days and noticed some large rust looking stains, distributed under the tree. I’m guessing these are bird droppings. Well, they have proven quite resilient to my attempts to remove!

I have attempted mild dish soap, vinegar, hydrogen peroxide, and another grease stain removal containing TSP among other proprietary ingredients.

I am at my wits...

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