Gray Fur-Like Growth Under House: Likely a Fungus

The grayish, fur-like mound (3–3.5 inches tall) growing on bare soil under your Marin County home is almost certainly a fungus (mushroom/toadstool)—possibly a species of Coprinus (inky cap) or a slime mold in its fruiting stage.

What to do:

• Removal (as you did) is fine. Wear gloves.
• Improve ventilation and drainage under the house to discourage recurrence.
• Don't worry—it's feeding on decaying organic matter (wood, roots) and will disappear when its food source is exhausted.
• If it reappears rapidly or spreads, consult a structural pest inspector for moisture/rot issues.

It's not a plant, not harmful to the structure directly, but indicates damp conditions worth addressing.