some mold is toxic, main concern is some of that getting into the tankI let rock sit outside in a bucket and re-used it. I even used a extra large coral skeleton from my moms garden outside (was decretive). No issues. I never bothered with acid or bleach. I just cycled it. I suppose worse case is it could leach out PO4 for awhile if it was from a dirty tank or in a garden for years previously.
but salt will kill the mold... it's not like going to come to life and eat your fish