Sarcoplasm is actually the cytoplasm of the myocytes (Muscle cells). It differs in composition as it contains a great number of ATP producing enzymes which are normally very low in a normal cell cytoplasm.
The name is different because it reflects what cells it is found in. Cytoplasm is found in all body cells but in this case, the prefix sacro- (muscle) has replaced cyto- (cell). That narrows it down to the muscle cells.