@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.
@thanghe
On Millim's "no da"; the two syllables she ends her sentences with. It doesn't really mean anything really, it's akin to accent, a childish one.
At first I didn't think of it as rude because many like to look their best once in a while even if it means wearing faux "feathers". Shit I'd say more than 50% of women wear makeup at least.