Responses
It's a bit confusing but I did a little bit of reading since I was curious too and apparently there's two terms that refers to asexuality. One refers to romantic aces (ノンセクシュアル - nonsekushuaru or 非性愛 - hiseiai) and one refers to aro aces (アセクシュアル - asekushuaru or 無性愛 - museiai). So it seems like they don't have a separate word for aromantic.
http://urasunday.com/shimanamitasogare/comic/47740.php
Here they use アセクシュアル if anyone is wondering.
Is there a word for aromantic in japanese? Because a lot of people seem to use aromantic and asexual interchangeably and they are not the same thing althought some can be both