Unbranded generally means generic, off-brand usually signifies it's branded, but not with a recognizable brand. Generic products are generally cheaper, but off-brand products can try to masquerade as higher-market items (especially when you're dealing with a purchase as infrequent as a mattress) and can claim higher margins.
What would you call something made in the 3rd shift? Where they use the same materials and factory they use for making branded gear, but don't put the brand on it?
Generic or unbranded, depending on how the product is labeled. The thing with those two labels is that, since you don't have someone actively trying to maintain those brands, you as a customer don't know if they're actually using the same materials and factory (or at least that's the theory behind why a brand would be worthwhile at all).