Using a mattress protector should keep your Simba mattresses in excellent condition, including the cosy, hypoallergenic cover. But if you do need to give the cover a clean, check which type of mattress you have:
For the Hybrid® with wrap-over cover: unzip the white sleep surface and pop in the machine at a maximum of 40 degrees (top half only!). You can also tumble dry at a low setting.
For the Hybrid® with the original cover (which doesn't wrap over the sides): use a light cleaning product on a soft, damp cloth in small, circular motions to spot clean and refresh.