Sounds like you're on the right track. If you think it's still cycling, unstable water parameters could be the explanation for the mold (if that's what it is). Your tank might very well have enough of a bio-filter to handle the fish you have (remember, during fishless cycling an enormous amount of ammonia is dumped into the tank at one time - way more than a couple fish could produce at once), but just to be sure I would suggest some extra water changes, at least until you get your test kit and can know for sure. Water changes will protect the fish from any potential ammonia/nitrite poisoning, and might diminish the mold as well.
Water molds don't handle salt very well, so if the mold persists after you know the tank is cycled, it will almost definitely go away once you add marine salt. Just remember to add salt very slowly if the figure-8 puffer is being kept in fresh water so they can adjust.