Your water temp is a little low which may be stressing the duster. Most of the Reefers on this site keep theit tanks temp around 78 -80. The brown algae will go away in time it is most likey diatoms which you will get while the tank is cycling. Also if not already try switching to RO/DI water...
More info on your setup would make it much easier for someone give you a proper answare. Such as how long has your tank been setup and what are your current water parameters (PH, SG, Ammonia, Nitrates, etc.)
Rob
With the amount of LR and LS you could go with a cpr backpack skimmer which hangs on the back of the tank. A lot of people are having great sucess with them and they are easy to setup and maintain. I had one on my 29 gal fowlr before I moved up to a 75 gal reef with built in overflows.
I haven't had any trouble with mine either. Haven't removed it from the tank since I got 2 yrs ago. Have more trouble with the Mag return pump restarting after a power outage.
The CPR units are in the $70 -$100 range depending on th size. Did see some post on one of the RAGS about people having problems with them over flowing. I was looking at them and have decide to put one in the new sump that i am plannong for a winter project.