Pearlwort (Sagina procumbens) with possible Procumbent Yellow Woodsorrel mixed in

Sagina procumbens

What's visible: The photo shows a very low-growing, mat-forming weed with thin, needle-like leaves radiating outward in a star or rosette pattern from central nodes — this is the classic look of pearlwort, which is extremely common in PNW lawns especially in partial shade and damp conditions. The tiny tan/brown seed clusters visible at the center of the rosettes are the spent flower/seed heads of pearlwort. There's also what appears to be a broader-leafed plant with scalloped edges in the upper left, possibly a small clover or woodsorrel, but pearlwort is the dominant invader here. This is NOT crabgrass or nutsedge — the needle-fine leaves and low mat-forming growth rule both of those out entirely.

Why it's there: Pearlwort thrives in exactly the conditions you have: the cool, wet Pacific Northwest climate, partial shade, and lawns with thin grass, moss, or thatch buildup (visible in your photo). It spreads by seed and creeping stems and can quickly take over bare or weak areas. Your ZIP code (Gig Harbor/Key Peninsula area, zone 8b) gets significant spring rainfall that keeps soil wet — perfect for pearlwort. The good news is it responds well to broadleaf herbicides and improving lawn density so grass can out-compete it.