sounds like the start of a fungal(or bacterial) infection. dont stress it, yet...your new tank is cycling, so....watch them everyday for signs of illness and stress and changes. every few days tank a sample of water from tank and get it checked at petsmart or petco(or simular friendly chain that has this service for free) and make sure your ...
... ammonia levels should rise, never above .25 ppm(not safe level) or do a partial water change of 10-25% when levels exceed .25ppm. now a week or so later of keeping ammonia at safe levels your nitrIte should start to rise. this is good sign of cycle starting . keep checking water paremeters every few days and these levels need to stay in the lower to stressed levels while it starts to grow nitrifying bacteria.if your levels of ammonia get to high,(over .25 ammonia) always do a SMALL PARTIAL WATER CHANGE. do not CLEAN your gravel during this time and feed the fish sparingly to help keep ammonia levels low.by week 3(or sooner or later) you will hopefully get a nitrIte (again) spike, it will go very high, do not worry more than usual...lol...it is gonna drop soon and you should see your first signs of nitrAte as your nitrIte level spikes and then drops down to safe levels.when your nitrAte level rises (10-40 ppm) and your ammonia and nitrIte levels drop to ZERO....YOU HAVE COMPLETED THE CYCLE. HURRAY! YOU NOW CAN RELAX and clean small portions of gravel and do regular partial water changes monthly or weekly( depends on you and your tank ) . hopefully your fish slowly got use to the climbing levels of ammonia and nitrite and you did not add more fish when the levels were rising ...fish adjust to ammonia, slowly... and will shock if you buy a new fish that was just coming from a established(cycled a long time ago) tank and you place in your tank with high ammonia levels. (big no no)...now if you have white spots or fluffy tufts on fishes body or other signs of illness you should treat them at this time according to the symptoms. always quarentine new fish for a few weeks for now on, so they dont bring disease into your tank you have been working so hard on...lol ...hope this helps you. good luck, dont be afraid to ask !
-------------doogle