A fast-growing marsh grass cropping up on the shores of San Francisco Bay looms as one of the nastiest invaders ever to hit the tidelands, experts and government officials say. Introduced innocently in the 1970s to check erosion in the southern reaches of the bay, the grass is building toward a growth explosion that threatens to choke much of the bay unless landowners join to stop it, say officials and researchers studying the problem. Wetlands experts working with the California Coastal Conservancy are leading a campaign to stamp out the existing 500 acres of the grass, Spartina alterniflora or East...