This is called hashioki, chopstick rest. It is used to keep the chopstick tips off the table. It comes in so many shapes, like the mushrooms and squid (ika) below.
Now that is the real thing, with an iron-based underglaze, an earthy and natural look.
Such a delicate pattern, I bought both in pink and green.
Karatsu Castle was known as Maizuru-jou, Dancing Crane Castle.