How does one cook Banh Su Xue?
It's hard to tell from your photo but if the báhn xu xê cakes were cooked, the outer green part would be fairly translucent since it's mainly composed of tapioca flour. Tapioca cooks clear. Yours looks cloudy still.

Báhn xu xê cakes are steamed for 15 minutes and then cooled. I've seen places where they're steamed in plastic wraps and I suppose that's safe but not what I'd consider appetizing. I honestly can't say how well they would hold together without some kind of wrapping while steaming. Traditionally, they were wrapped in pandan leaf folded into a box shape. Pandan leaves are used for flavouring as well as wrapping. My best suggestion is to rewrap them in double thickness parchment baking paper if you don't wish to use plastic wrap and tie with string to hold together. The paper will get soggy, of course, but should hold their shape.

By the way,  báhn xu xê translates as "husband-wife cakes" and is traditionally served at weddings.