What happens is that a bit of fixer is left around the outside of the spout. On standing for a time the water eveaporates leaving crystals. Wipe the spout off with a moist cloth and rinse the cap each time before you close the bottle. This should stop the problem.
Be careful that none of the crystals gets of the floor or any liquid fixer. You don't want fixer dust floating around the darkroom. Should it settle on paper or film there will be a problem.