My mom calls it 番薯芋 (she pronounces it in 闽南语: "han zi or"), this name refers to the Okinawan Sweet Potato's likeness to the purple yam.
When she talks about it in Mandarin, she calls it 日本番薯。
I wonder what other names it has in other places outside of Singapore.
The Chinese Wikipedia mentioned "紫心番薯". Baidu says "紫心番薯" is commonly found in Guangdong. Since one of the major characteristic of Okinawan sweet potato is its purple flesh, I wonder whether 紫心番薯 is Okinawan sweet potato, or whether it refers to another kind of purple flesh sweet potato.