Green pepper often means an unripe bell pepper, especially in US recipes. Elsewhere, it may refer to a green hot chile. In sauces and seasoning descriptions, it can refer to green peppercorns.
These are not interchangeable: sweetness, heat, size and texture differ substantially. Check the recipe's cuisine, the quantity and the preparation instruction. A sliced whole pepper and a teaspoon of peppercorns clearly require different ingredients.
Reference: Further reading.