Welcome to the Bunnings Workshop community @DeckingMJ. It's a pleasure to have you join us, and many thanks for your question about cleaning a deck.
Giving the deck a good scrub down with Cabot's Deck clean and a stiff bristle brush should remove the mould or algae buildup. Remember your PPE, including safety glasses, a face mask, gloves and a long-sleeved shirt and pants. If you have a high-pressure sprayer, that can also do a decent job, but make sure not to place the nozzle too close to the timber, or it can damage it. You'll need to re-oil your deck after the cleaning process.