Is it white? If so, it's a bacterial bloom.
There's a good bit of confusion and misinformation on this, so here's the rundown:
Since it sounds like you just filled the tank and added fish, your tank will be in a cycle as nitrifying bacteria build up to process ammonia and nitrite. Review this thread for the basics there:
http
/www.fishforums.net/index.php?showtopic=175355
The bacterial bloom, however, is a different type of bacteria. These normally live in the biofilm that coats all surfaces of the tank. They don't process ammonia, but process fish waste and leftover food, releasing ammonia. These reproduce much faster than the nitrifying bacteria, so when you have an excess of fish poop or uneaten food, they start to grow like wildfire, causing a bacterial bloom in the water. Since a new tank has no biofilm yet, this is fairly common for your situation. Nitrifying bacteria don't reproduce fast enough to do this.
The bloom itself is harmless, however, they're generally linked with ammonia spikes as they accellerate the breakdown of waste, and this is bad. Since the tank isn't cycled, you'll have excess ammonia to start with.
Review the thread above. If you don't have a test kit, start doing 50% water changes a day anyway until you can get one. Remember to use dechlorinator (I recommend Seachem Prime or a pond dechlorinator, as these use a drop or two per gallon and thus are far more economical than brands like Aquasafe). Gravel vacuuming can help deal with the bacterial bloom, as well, as it will suck up fish waste and food that would otherwise fuel the bloom. Don't panic if water changes don't fix the bloom, since the bacteria can grow fast enough that even after a 90% water change, they'll be back to full in an hour. Just focus on keeping ammonia and nitrite under 0.25 ppm.
Do you have your own test kit? You'll need one, liquid based are far more accurate and much cheaper in the long run than strips. Pet stores aren't great for doing water tests - they may tell you your water is fine when it isn't fine, but simply not of immediate worry. It's also inconvenient to use pet stores for monitoring, and you'll want to test regularly, especially during the next critical few months. During the cycle, you'll want to test at least once, preferably twice a day.
Some information that will help give far more thorough advice:
Tank size
Filter
Current live stock
Detailed water stats: ammonia, nitrite, nitrate, and pH.
Edit: If you don't have fish yet, disregard all of the above, it's probably just dust out of the substrate. If this is the case, please review this thread before you get fish:
http/www.fishforums.net/index.php?showtopic=113861
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