If you haven't backwashed your sand it wouldn't hurt to do it and may be needed. Backwash it until dirty water stops coming out, rinse, backwash again, and rinse.

Did the filter work ok last year? If so an algae episode of a couple of weeks shouldn't affect the sand to the point of replacement, if at all after it is backwashed.

If your numbers are good it could take some time for the water to clear. It took mine about a 1 1/2 weeks to go from cloudy blue to crystal clear. It is also very important to make sure you get all of the junk off of the bottom by a manual feed vac, or whatever means you have. I would also suggest using a skimmer sock (some people use pantyhose) as they work well for catching small debris.

I wouldn't change the sand yet and I certainly wouldn't be replacing the whole system. Cloudy water can mean many other things other than a filter replacement. Be patient with it, keep your numbers right by following Al's advice, and remember to run the filter 24/7 until all problems have ceased.

Good luck.