Dreaded Yellow Fuzz growing on petrified coral reef rock

[TexasDefender]

I bought the following from petsolutions dot com to add to my 50# of live sand in order to get my new 29 gallon salt tank up and cycling: "Reef Rock: A spectacular addition to fish or reef tanks. Actual petrified coral! Reef Rock is ideal for use as a base rock on which to attach invertebrates such as anemones, soft and hard corals, etc. Clean and pure, not bleached or chemically treated. 50 lb. box." About 2 weeks ago I positioned the coral rocks on my 2-inch-thick, week-old live sand and started waiting out the sacred month before buying fish to allow the tank to cycle etc. Well, my coral rocks were pure white until a couple days ago, when something started growing. Unless there is a yellow fuzzy coral growing out of the old holes in my supposedly dead (?) coral rocks, I apparently have "tank rot." Would someone here who knows much more than I do look at the two attached photos (showing white petrified coral with yellow fuzz growing on it). Should I be glad anything is growing? Or should I go on a crusafe to kill the yellow fuzz? I don't have chemical test kits yet, but my tank is at 79 degrees Fahrenheit, and 1.023 salinity.

To anyone who takes the time to look and respond, thank you very, very much. This is my first venture into saltwater and I don't want to have a toxic tank.