@Syapaaye and
@Nanaten have already alluded to the joke, but I thought it best to explain it for anyone who didn't get it.
Japanese custom at New Year is to give a gift of New Year's money,
otoshidama (お年玉). But the final kanji in that phrase, 玉, can also mean "ball" or "bead" or "jewel" (or "testicle" 🙂 ), so Rimuru is making a joke by giving them a New Year's ball/bead (thankfully, he didn't give them a testicle 😉).
It's a very lame joke, deep in Dad Joke territory, but it's one of those lame jokes everybody makes at some point.