Why is it that when you leave a bar of soap made from a melt and pour base out in the open it ends up covered in water droplets?

This applies to all soap bases and tends to be more noticeable depending on the humidity in the atmosphere.

This is what is often referred to as "sweaty soap" and it can easily be avoided.

So firstly why does it happen?

One of the main ingredients in a melt and pour soap base is glycerine.  Glycerine is a humectant which means that it attracts moisture.  So when you have a cream or lotion that contains glycerine, that glycerine stops your skin getting dry as it attracts moisture.

When it's in a melt and pour bar of soap it will also attract moisture from the air.  So the droplets you see on the bar of soap are actually droplets of water.  If you leave a bar out overnight it will soon be covered in water.  This is more noticeable in the warmer summer months as there is more moisture in the air.

So how to avoid it?

This is the easy bit as you simply need to cover up the surface of the bar of soap.  As soon as your bar is set and hard, simply wrap it in cling film.  This will avoid the problem of "sweaty soap".

Does it mean there is anything wrong with the soap and can I still use it?

No - the soap is still perfectly usable. Simply wipe off the droplets and wrap your soap.