I would say cut down on your lighting to about 5-6 hours for now...normally 8 hrs or so is plenty.
As far as your water change...your LFS advice is a little misguided. The benefitial bacteria that colonizes your tank will grow and thrive in the surfaces found within your tank i.e. your live rock, live sand, powerheads, etc. The water itself contains a miniscule percentage of the bacteria that you want in there, therefore routine maintenance water changes will not affect anything negatively.
To get rid of the cyanobacteria you can siphon it out (using tubing, gravel vac or even a turkey baster). In addition to cutting back on your lighting you should make sure you are not overfeeding.
As far as the flow...the water movement within your tank can still be inadequate with 2 powerheads, it all depends on the gph rating of your poweheads (how much water they actually push) and also how you have positioned them in relation to the rock landscaping and overall setup of your tank. Avoid dead spots and you will see a big decrease in algae growth.
Good luck!