The deck is treated wood deck about 8 years old. Every 1 to 2 years I
cleaned it. This year due to mold and mildew I cleaned it with bleach, and
then pressure washed it. A white residue is left after the deck dried. it
looks almost like salt. How do I get rid of the residue before I put the
preservative on?

Wont it wash off, It could be the CCA treatment Arsenic, nasty stuff.
Keep your pets off of it and scrub it off.