For a moment I thought SLBM.Some are sub-launched (submarine-submerged), known as a submarine-launched ballistic missile (SLBM). Submerged launches are not considered surface-launches. Americans also experimented with air-launched ballistic missiles (ALBM) called GAM-87 Skybolt, but abandoned the programme.
But I do understand that there is no point in referring to it as SSBM. The lay audience wouldn't understand the terminology, even if you use the proper terms like SRBM, MRBM, IRBM, ICBM, SLBM, ALBM, GLCM, SLCM, ALCM, etc. In fact, that might be more confusing to them. The target audience, however, is quite learned and needs little to no explanation on the classification of the concerned missile.