Pervasive Winter Lawn Weed
This appears to be a Chickweed (Cerastium or Stellaria), a common weed in lawns and flower beds in our region.
Physical removal is the most immediately effective technique for control, but if the colonies are too numerous or extensive, herbicides may be needed. If yours are in lawn, a post-emergent broadleaf weed control product (there are too many to list) that mentions on the label that it is safe for use on lawns should work. Pre-emergents are best used in fall when winter weeds such as this begin to sprout. As they are annuals, they will live less than a year, though you do want to prevent the seedlings from gaining a foothold and perpetuating the problem. Lawns with weeds tend to have less-than-ideal turf vigor, bare patches from prior weed colonies, and other cultural issues that can be easily corrected. Having a soil test done [https://extension.umd.edu/hgic/topics/soil-testing] can help illuminate any issues with pH or nutrient excesses or deficiencies. Overseeding grass in fall to minimize bare patches, irrigating during drought with deep, infrequent waterings, and mowing high with sharp blades all help maintain healthy turf that can out-compete most weeds. Lawns should also receive at least 6 hours of sun per day in summer; if they do not, they will continue to struggle and alternative groundcovers should be considered.
If the weeds are in flower beds, hand removal may be the safest option, with a pre-emergent labeled for use in flower beds applied in fall to prevent the next crop of seedlings. Herbicides are more challenging to use in beds of mixed desirable plants as any accidental exposure may cause damage; some may be cosmetic only, but serious damage can occur depending on the chemical being used and how the plant was exposed. Spot-spraying or painting on the herbicide with a dedicated brush or sponge is one way to target applications in tight quarters. Using either bark mulch around plants or planting another ornamental ground-covering perennial to serve as a living groundcover are the best ways to avoid open soil where these weeds can become established.
In either case, if herbicides are used, their timing will partly depend on which chemical is involved. The package label should have all of this information, including application interval (if more than one treatment is needed) and timing of application. Even though we are currently experiencing a warm spell, typically the weather needs to be more conducive to treatment for a longer period. Plant metabolism changes with the weather and the plants may not succumb to the herbicide as quickly or completely if the weather changes too quickly. Weather can also influence how quickly the chemical breaks down in the environment from exposure to water, light, and soil microbes.