First, let's work with and define three very closely related terms: neoplasm, tumor, and cancer. A neoplasm is a growth mass on tissue that arises from abnormal cell growth and division. A tumor is a neoplasm that has formed more of a lump type shape as opposed to a general, shapeless growth. In modern medicine, the terms neoplasm and tumor are considered to essentially be synonyms of one another. Cancer, on the other hand, is uncontrolled cell growth and division that is malignant, meaning that it spreads and invades other tissues in the body (metastasis).
The important thing to keep in mind here is that these three terms are not interchangeable and do not all indicate the same thing. Neoplasms can be divided into three categories: benign, potentially malignant, and malignant. Benign growths are highly localized and do not pose a serious heath risk. They can usually be removed to solve the problem. A potentially malignant neoplasm can become cancerous in time but if found and treated early enough can be treated and removed before the condition becomes life threatening. Malignant neoplasms are cancerous and are indeed life threatening. They must be treated with more drastic measures like chemotherapy or radiation to destroy all of the cancerous cells before they spread too far. The problem here is that healthy cells are also killed too, so the treatment is very harsh.
So the ultimate answer to your question is that no, not all neoplasms are life threatening.
The cancer cells are altered self-cells that do not need to normal growth regulating mechanisms; as a result, they continue to divide and produce a tumour or neoplasm.
"Neoplastic cells undergo rapid, abnormal and uncontrolled growth at the cost of remaining cells are called . The growths resulting from the division of such cells are called neoplastic growths or tumours. Tumours can be commonly classified as benign and malignant. Abnormal and persistent cell division that remains localized at the spot of origin results in the so-called benign tumours. It should be noted, however, that benign tumours can sometimes be fatal, e.g, brain tumours that cause pressure on vital centres."
Benign tumours are incapable of indefinite growth and invasion of healthy surrounding tissues and usually contain well-differentiated cells. Malignant tumours have the abilities to grow indefinitely and invade surrounding healthy tissue and usually contain undifferentiated cells, often with large nuclei and nucleoli.
Powar,C.B. (1999). Cell Biology