Could be the particles arent all getting caught by the filter during vacuum, and they are settling down overnight. Then you wake up to "more" algae, which is just stuff that was stirred up or pushed thru the filter and settled down again.

Best bet is to vacuum to waste. Ive altered my opening procedure to take this into account.......I shock the heck outta the water and let it all circulate for 48 hours. Then I do a vacuum to waste to get everything off the bottom. Instead of vacuuming first, which just seems to stir stuff up and have it re-settle.

-Chris