Phyllospadix iwatensis (a species of seagrass)
A specific marine plant native to coastal waters around Japan; the name is typically written in katakana in field guides and everyday use.
Sugamo is a seagrass found along the coasts from Hokkaido to Kyushu.
The seabed around here is covered with sugamo.
The name likely derives from the plant's resemblance to sedge (菅, すげ) and its aquatic nature (藻, も), but the exact origin is uncertain.