The waste water that leaks out is water that already now goes into that coastal environment, and the algae that leak out are biodegradable, and because they're living in waste water, they're fresh water algae, which means they can't live in salt water, so they die.
漏れ出す排水は 元来そのまま 排出されていた排水ですし 藻類は 漏れても自然分解されます 排水中で生育する藻類は 淡水生物なので海水の中では 生息できないのです