Let it go to seed and then you'll have a bunch of regular old coneflowers come up next year. I like Ruby Star. It has such big blooms!

I'm having the same trouble with my plants this year. My two are clear on the other side of my garden from each other. I'll be tagging the pink ones so I can remove them from the patch of white and find them a different location on the property. See what they end up being. I really do favor my white babies though. The Shasta daisies don't last long enough, and get super leggy in this area. That's why my white Echinacea are my most beloved. I'm sure that they got mixed in by the birds. Not sure if you are aware, but much of random reseeding in gardens is from bird droppings. The birds are Mother Nature's best gardeners. I have Alchemilla that is only in the front of my property , and find baby seedlings in the main perennial garden out back. Helps when I want to add to the front, but don't want to divide yet. Lol.... Feels like Christmas every Spring!

ID is easier, and more accurate, if include flowers. post pix of the flowers when they arrive.

Just keep flushing the area well with water. This will dilute the salts to the point where they will have minimal adverse impact. And be careful how you use mushroom compost. It is best if used to amend soil well in advance of any plantings. "There are several uses for mushroom compost. It can be used as soil amendment for lawns, gardens, and container plants. However, this product should be used with caution due to its high soluble salt levels. These salt levels can kill germinating seeds, harm young seedlings, and cause damage to salt-sensitive plants."