When temizu is too much, the mochi is soft during the pounding, however, mochi tends to become hard for stretching and shaping conducted later and also tends to become moldy later on.
手水が多いと餅を搗いている最中は柔らかいが、後で延ばしたり成形するときに固くなりやすく、先々カビが生えやすくなる。