Ok, chatgpt got my back (so take with a pinch of salt). It's called a fairy ring and the greener grass is from:

1. Increased Nitrogen Availability: The mycelium decomposes organic matter in the soil, releasing nitrogen and other nutrients, which promote greener, healthier grass growth.

2. Enhanced Water Retention: The dense network of fungal hyphae can improve soil structure and water retention, providing better moisture availability to the grass.

3. Soil Aeration: The fungal growth can improve soil aeration, helping grass roots to access more oxygen and grow more vigorously.

4. Hormonal Effects: Some fungi produce hormones that can stimulate grass growth.