Cold closet with air brick inside -- can I insulate it somehow?

[Mander]

My house is a 1950s brick semi-detached, with plaster directly on the brick. There are a few air bricks (right term?) around the house, including one that is inside a closet on the upstairs outside corner of the house. As you might imagine, this closet is always cold and has a tendency to grow mould in the corner, which I presume is because the door is usually closed so warmer, drier air from the rest of the house isn't circulating there. The plaster is cracked in interesting ways which I hope is also a result of the cold inside, and not a bigger structural issue.

Any suggestions for making the closet less cold? The exterior walls are themselves insulated but I was thinking of putting something on the wall inside the closet or maybe sealing off the air brick somehow.

 

[foxhole]

Mold would suggest vent is blocked as ventilation prevents mould spores settling .

 

[Mander]

I don't think it's blocked as there is a cold draft inside the closet. I was thinking that the mold was the result of the cold air coming in but there not being any real circulation in there.

 

[foxhole]

Mould spores exist all over the house they only settle in cold draft free area’s.