We tend to put a lot of stuff on our decks and sometimes these objects lead to unsightly marks. The most common is a deck chair or barbeque leaving a rusty or black ring on your deck.
What has happened is the metal in contact with decking has oxidized or rusted and left a mark. To test for oxidation get a small amount of Oxalic Acid and apply to the stain. If you have red or black rust on your deck Oxalic Acid will almost instantly reduce the appearance of the mark.
Oxalic Acid is normally sold in a crystalline solid that can be dissolved in water and sprayed onto surfaces. Spraying Oxalic Acid directly onto your stains will remove the stains but it can also leave a lighter patch where the Oxalic Acid has bleached the timber. If you are going to treat your deck with Oxalic Acid then it is better to apply it to the entire area and just repeatedly spray the areas most badly affected by oxidation. Oxalic Acid also acts as a mild bleach and can help in cleaning away marks left by plant pots and other objects left sitting on the deck.
Deck chairs and plant pots are not the only things that can cause black oxidation on decking. Decks can be affected by environmental pollution and other activities. If you cut any metal near decking the iron fillings can rust on your deck and appear like mould but in fact it is a rust issue. Tin roofs can leach iron into rain water if it splashes over the guttering onto decking. Decks near industrial activity can suffer from air pollution raining down and leaving iron pollutants on the timber.
It is quite important to test what you are dealing with before trying to clean your deck. What can appear to be mould will not be cleaned away by anti-mould cleaners if it is iron oxidation. Be aware of this if you have had your roof washed, your house cleaned or had building activity around your deck.