Mammillaria melaleuca flowers are remarkably similar to M. sphaerica's. It has a very limited range, coming from Mexican state Tamailupas where much more widely distributed M.sphaerica grows as well. These two species are probably closely related.
Mammillaria melaleuca is relatively easy to grow plant, the largest one I have is about 10cm across and still singular. It's blooms usually form classical Mammillaria 'crown'.