Been there...done that....fixed it.
Spray WELL with undiluted white vinegar
to kill the mold spores.
Let the area dry completely
, then dry brush to remove as much of the residue as possible.
Follow by cleaning the area with an OXYGEN
bleach (e.g., "Oxy-Clean"--mixing & application directions are on the container).
Assuming the canvas was totally dry when you closed the bed-end (and no sneaky little bits of material were sticking out), you probably ought to check the seal, too.