Yes, but... ...Not always a cancer. Different types of polyps have varying risk of developing into cancer or harboring cancer already. Size also increases the risk.
Some yes, some no. Neoplasia is a general term to describe abnormal growth pattern, cancer-like, unregulated by normal bodily control systems. Some polyps are destined to become cancerous, some are not, so all polyps are usually removed when they are found so we can distinguish the difference under the microscope, not by appearance which can fool you. Not an area to make a mistake, i assure you.