Would standing next to a pool of mercury do you any harm?
It depends. But there is little short term risk.

Mercury metal isn't very toxic. You can even ingest it without significant short or long term effects. But mercury vapour is nasty and you don't want to be continuously exposed to any significant amount of that (see this answer for more detail). And some mercury compounds are pretty nasty too.

Of course mercury metal emits mercury vapour so there will always be some in any room with exposed mercury. But the reason why this isn't an immediate worry is that it takes time to absorb enough vapour for it to become notably toxic. Chemists are used to handling liquid mercury in the open lab but are a lot less comfortable if it has been spilled and not cleaned up. This didn't used to be so which is one of the reasons we know the dangers: chemists who were exposed to vapour over years did suffer toxic effects. Mercury miners suffered similar long term toxic effects from the vapour. In modern labs we make sure the vapour doesn't build up by good ventilation and making sure any spills are cleaned up quickly so exposed mercury doesn't hang around emitting vapour.

So would you be in danger standing next to a pool of mercury? No, if you didn't do it for long. Or, if the ventilation in the room were good and the vapour wasn't allowed to build up. Even then, though, modern safety standards wouldn't be comfortable having pools of mercury exposed to the atmosphere: you wouldn't want to regularly work in that room with the pool. 

In short, standing next to a pool of mercury even in a badly ventilated room wouldn't cause immediate harm; working next to that pool for months or years would certainly cause you harm.