What is unusual about the currency used on the island of Yap?

It's made of stone & as large as 12 feet high.

https://www.bbc.com/travel/article/20180502-the-tiny-island-with-human-sized-money

https://en.wikipedia.org/wiki/Yap#Stone_money

It is stone money. Although the production has been ceased in 1931, it is still in use for purchase of land. The stones can weigh up to 5 metric tons and are not moved. People just memorize who the current owner is.

The stones are made of aragonite and calcite, two minerals which do not naturally occur on Yap. So stones have to be brought from Palau, an island about 400 km away, which requires a boat trip of 5 to 7 days.

The value of each stone is determined by a series of factors: size, form, natural beauty of the material, age and history of the stone, difficulties in manufacturing, social status of the people involved or simply whether someone has been injured or died during production or transport.

https://en.wikipedia.org/wiki/Rai_stones

What's already is answered is correct. Two special cases, though:

There are "coins" that are smaller and movable. These are kept around the home while the large ones are in "banks", which are clearings in the woods.

It happened that the boats that brought the stones to the island sank just before arrival, with the cargo. Such places on the bottom of the sea are also valid as "banks".