It seems to be concrete but not sure whether how this can be cleaned, tried to clean with pressure washer but not much effective. unsure whether this can be repainted, pls guide. Also, what material can be used to fill in the gaps.
Welcome to the Bunnings Workshop community. It's brilliant to have you join us, and thank you for sharing your question about your flooring.
From the images you've shared it seems like the original pavers had a smooth finish on them. Time and overcleaning have stripped away the original surface finish allowing moss and mould to settle onto the surface of the paver. In order to clean the surface, I propose using 30 Seconds 5L Outdoor Cleaner Concentrate in combination with Oates Long Handle Deck Scrub Brush. Please make sure to give the outdoor cleaner enough time to activate before you start scrubbing the surface. For an extra clean finish, I suggest using your power washer to wash away the cleaner.
To cover the gap between the pavers, I suggest using gap filler rod in combination with Gripset Betta 1L Concrete Crack Filler. Place the gap filler rod between the cracks and then fill the gap with the concrete gap filler.
To seal the top of the paver you can use either Crommelin 4L Natural DiamondCoat Paving And Concrete Sealer or paint over it using Berger Jet Dry 4L Heavy Duty Deep Charcoal Paving Paint. Please make sure to follow the instructions when preparing the surface of the paver.
Please remember to wear personal protection such as gloves, and goggles when using the 30 Second Cleaner.
If you have any other questions we can help with, please let us know.
It's great to see that you have already received some excellent advice from my colleague @EricL. I thought I'd point out that the surface is likely stamped concrete. Doing a test section with the 30 Seconds spray in an inconspicuous area is best to ensure it doesn't react adversely with the stencilling paints.
Please let us know how you go and if you have further questions.