Yes—if mold has already found something to feed on, such as the gelatin layer of the film, it may continue to damage the slides unless you clean and sterilize them first.

Based on the advice given, the key steps are:

• Clean the film with dry/minimal-water isopropyl alcohol.
• If the slides are in cardboard mounts, replace those mounts, since cardboard can retain mold spores.
• Keep contaminated or badly damaged slides separated from clean ones.

The exact safest treatment can depend on the film stock, but Kodak specifically recommends dry isopropyl alcohol for this kind of cleaning. Simply moving slides into archival boxes helps with storage, but it does not by itself remove mold spores already present on the film or mounts.

So for long-term archiving, the safest approach is to clean the film, replace any contaminated cardboard frames, and only then store the slides with the rest of the collection.