Invasive lawn weed
This is wild violet. It's a tough perennial weed to eliminate, but its flowers can actually be useful to wildlife.
Here's our page about managing it. Early fall is the best time to get serious about some of the tougher lawn weeds. https://extension.umd.edu/hgic/topics/wild-violet
Keep in mind that a perfect lawn is difficult to achieve in MD without a great deal of chemical application which doesn't happen without cost and environmental impact. Regular fall fertilization and mowing high are the two premier simple strategies that benefit lawns.