Several of the fifty or so species of Yucca are difficult to distinguish from similar species. Some are varieties of species, rather than distinct species. Some are naturally occurring hybrids. Curve leaf yucca, Yucca recurvifolia, is supposedly a naturally occurring variety of mound lily, Yucca gloriosa var. recurvifolia (or tristis). Alternatively, it could be a hybrid.
As if that is not confusing enough, its physical characteristics are variable. Foliar color is typically grayish, but might be simple olive drab like that of many other species of Yucca. Its typically pliable evergreen leaves that curve downward as they mature can be almost as rigid as those of common mound lily. Stout but upright trunks may or may not develop.
Curve leaf yucca is remarkably resilient. Actually, unwanted specimens can be difficult to eradicate. Small bits of rhizome can generate pups for many years after the removal of a primary plant. Occasional watering is appreciated through the warmest summer weather, but may not be necessary. Old colonies can get ten feet tall, and occupy significant area, but quite slowly.