Angel moths prefer eating wool and other textiles, but since there is no setting for textiles, it seems to ignore it when provided. The description does mention that it will eat grass, and its stats page lists kibble and other food types as well.

My experience is that they just starve until hospitalized, where a colonist will be able to feed it Kibble etc.

Tried giving it specific rules to be unrestricted, but still wont eat anything on its own.

My guess is that they regularly "demand" textiles, and in between those urges it eats other stuff, but I have no idea.

Alpha Animals seems to be adding a lot more funky creatures with weird diets, which I guess could cause further future incompatibilities.