A composite deck is made from a mix of wood and plastic. There are many different brands of composite decking--each are a little different (better or worse) regarding scratch- and stain-resistance, appearance and durability. No matter what brand you own, however, if you spill grease on this decking you should clean it off so that is does not leave a stain. Hopefully, you'll notice the grease spill soon after it occurs. The longer you wait to clean it up, the longer it will generally take to remove.
Put on rubber gloves. Wipe up as much grease as you can with paper towels.
Spray the deck with a garden hose (turned on high) to remove more of the grease.
Mix hot water and liquid dish detergent in a bucket. Use 1 tbsp. of detergent per gallon of water.
Wet a nylon bristled brush in the solution. Scrub it over the greasy area to lift the grease off the decking.
Saturate a sponge in the soapy water. Wipe this over the grease. If the grease is not coming up, pour liquid dish detergent directly on the stain and let it sit for 15 to 30 minutes before scrubbing it again with the sponge.
Sop up more grease and soap with paper towels. Spray the deck with a water hose to remove soapy water and the loosened grease. If some grease remains, wash it again with soapy water until all traces are gone.