a plain good ol' もち tossed in green soybean flour (うぐいすきな粉). However, the 求肥 dough is coloured slightly green, so it looks like little happy birds waiting for spring.
In terms of taste, it's pretty normal tasting: もち dough around a core of 粒あん (25 gr is a good size here). The きな粉 dries it out a bit, so don't overdo it with the dusting (and absolutely don't dust the 中餡!).
When shaping, press the 中餡 through the 求肥 disc, then press the ends together to enclose it. Press slightly to make a diamond shape, then dust with more きな粉.
Ideas
I coloured it with some pandan powder, and I think that's a decent enough way to add a little bit of flavour and a beautiful shade.
I think some 山椒 might be interesting, as well as lime zest or green wheatgrass powder