More info on the 宝石人魚姫 ~jewel mermaid ~ noctchill collab at Gamers
Small IS and Alstro mermaids are also available. Purchases of 2,000円 give a postcard.

https://idolmaster-official.jp/news/01_16119
https://twitter.com/gamers_no_gema/status/1953637876874588175
https://twitter.com/imas_official/status/1953638556167291046