Plagiomnium vesicatum

A mat-forming moss from temperate East Asia, Plagiomnium vesicatum grows naturally along stream banks and waterfall margins in Japan, Korea, and China — and is sometimes found growing directly in shallow water. Its broadly elliptic leaves curl tightly when dry and flatten out when moist, making them a reliable indicator of the terrarium's hydration level. It spreads as a dense, low carpet in shaded conditions and does well in the enclosed, high-humidity environment of a closed terrarium.