あかもくじゃなかったです
赤藻屑じゃなかったです
akamokujanakattadesu

Noun
Desu Copula

The polite, past negative form of あかもく

1. It wasn't akamokusa.; It wasn't red seaweed.

Past/Perfective
Negative
Polite
Casual Register
Usually Written in Kana