These two ought to be kept separate, "mucus" for the slimy substance and "mucous" for the adjective describing whatever contains or secretes mucus. So "mucus membrane" is a mistake -- e.g.: "If a person already has herpes, chlamydia, gonorrhea, or syphilis and is exposed to HIV, he or she is more likely to contract the virus because of openings in the mucus [read 'mucous'] membranes and skin." "Increased Research Leads to Dramatic Declines in Sexually Transmitted Diseases," USA Today (Mag.), Feb. 1995, at 11.
In medical literature, "mucous ball" and "mucus ball" are about equally common. This is a closer case than "mucous membrane," however, because "mucus ball" is a ball of mucus, and the noun is justifiably used in an attributive sense -- e.g.: "Over the ensuing six months, there has been no further problem with mucus balls, his level of dyspnea, or edema." Edward M. Harrow et al., "Respiratory Failure and Cor Pulmonale Associated with Tracheal Mucoid Accumulation [etc.]," Chest, Feb. 1992, at 580. But the best course, outside medical literature, might be to find a euphemism.
Ah, the "mucus ball." We hope your invitation was lost in the mail. Sincerely, and sniffingly, SPOGG.