Chemist here. Organometallic compounds of Mercury are primarily dangerous because Mercury ions bond fairly strongly to soft ligands like sulfhydryl groups found near the active sites of enzymes. This can result in the displacement of the metal ions or otherwise disrupt the structure needed for enzyme functionality. Mercury metal OTOH is considerably less reactive. It is not safe to breathe in for prolonged periods of time but it is no where near as toxic as its organometallic derivatives are. Dimethyl Mercury for example, is extremely dangerous. A single drop has 100+ times the organomercury content needed to kill someone.