When foliage falls onto the surface of the concrete and gets wet, tannin releases from the leaves and will stain the surface of the concrete. If the concrete is sealed with an acrylic, the tannin will stain the acrylic. The only way to prevent tannin from staining your concrete or coating, it to blow the leaves off your concrete as soon as you notice them, and do not allow them to stay on the concrete for long periods of time.
If your concrete develops tannin stains, there are few things you can do. First, you can leave the stains. UV light will bleach the tannin stains and they will dissolve naturally on their own over time. If you don’t want to wait for the stains to dissolve on their own, you can use a gentle non-acidic degreaser like ZEP Purple Degreaser, or a diluted bleach mixture. If you decide to use bleach, apply to a test area first because the bleach can change the color of your concrete.