Hi Newstrand1
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.
Good luck!