Is melanosis a type of cancer?

Melanosis Coli. I think you are referring to melanosis coli. This is a discoloration of the inner lining of the colon, which is not cancerous, but can make it difficult to notice irregularities.
NO. Melanosis is accumulation of pigment in different tissues for a variety of reasons. In the skin, it can be due to sun damage, after inflammation or injury or with long standing poor circulation. Melanomas are usually pigmented but are not termed melanosis. Melanosis coli is pigment seen in intestine of people who use laxatives for a long time.