Is there such a thing as a mucinous carcinoma breast cancer?

Yes: Mucinous, or colloid, breast cancers are a rare type of breast cancer characterized by the production of mucus within the tumor. These tend to be low-grade tumors, are usually estrogen-receptor positive and her-2/neu negative. Treatment follows the same criteria for all breast cancers.

Yes: Some carcinomas of the breast may make mucus and those, though uncommon, are referred to as mutinous adenocarcinomas.