Something is missing if it is not there, and it should be there. One of the chocolates in the picture is missing.
Mistletoe is a name for a group of unrelated plants that all grow the same way - by being partly parasitic on another plant. This means they grow partly inside another plant, using it as a source of nutrition. Mistletoes can also generate some of their own food from the sun like other plants.
Mistletoe is often used as a Christmas decoration. It is a tradition that any male and female who meet under the mistletoe must kiss.
Mixing is a way of combining two or more things together so you cannot easily tell what went into the mix.
You make cake batter by mixing the ingredients together, and you make new colours of paint by mixing different colours together. Food described as 'mixed' like mixed candy or mixed nuts is made from several different kinds of food mixed together.
The picture is of someone mixing several different colours of paint together.