So I guess my question is, is there a mold that would be beneficial to have grow in such a confined environment? I assume the ventilation system can filter out the spores, are astronauts by way of the enclosed environment subject to more exposure to spores than on Earth or are the numbers much lower by volume?
In Vernor Vinge's "A Deepness in the Sky", where ramships travel at sub-light speeds between the stars in the story, they use algae (IIRC, if not that some other similar thing, as opposed to big plants) to process CO2. The process doesn't just run in one big vat, though; it has to run in multiple partitions, because it's a routine event that a mutation in a particular partition manages to work out how to more efficiently convert the provided resources into reproduction, at the cost of putting out less O2 or putting out other undesirable gasses. It's a routine maintenance job for them to look at the output, take some gene sequence samples, and dump & sterilize sub-par partitions and restock them from the other cells.
I give this as an idea of the potential problems we'd face right now. Even if the tech to do that is a routine design job today, it's still not something we have (AFAIK) and would need time to be tested, and the result would be, as I said, quite bulky for some time (for instance, if you work out the necessary volumes, IIRC it's not like you get "20 cc of algae per astronaut" or some similarly dainty value; it takes quite a bit of algae to put out enough oxygen for our high-energy metabolisms).
[1] https://en.wikipedia.org/wiki/Opisthokont [2] https://en.wikipedia.org/wiki/Eukaryote
Might be worth combining threads.