I agree with your local winery supply store employee; it’s most likely a surface yeast or “flor” yeast of some kind, forming a floating plaque on top of your wine. Sometimes referred to as “wine flower” (or the Spanish word, flor), these yeast aren’t turning sugar into alcohol, they’re actually eating alcohol and oxidizing it in the process. It’s also possible that part of your “floaters” could be bacteria. In any case, the remedy is largely the same. Practice exacting sanitation, exclude oxygen from everything as much as you possibly can after primary and secondary fermentation are complete, and maintain appropriate levels of SO2 for your wine’s pH.
Higher pH wines need more SO2 to have the same effectiveness so if you’re over 3.65 I’d keep my finished wine around 35–40 ppm free SO2 if I knew I had something growing in it. Under 3.65 and you can get away with slightly less, say, 30–32 ppm FSO2. Since you say the wine tastes fine, I’d say that’s a very good sign that you might be OK. This may be a wine that you should filter before bottling. Using a 0.45 micron ‘sterile’ filter will assure that no bacteria will get into your wine bottles. You may also want to consider bottling it a little early to ensure it’s protected from oxygen.
