If u can pull it out u can spray with h2o2. You can also use excel. You could also do a bleach dip which I think is a 1:20 dilution with water and then cleaning with water overdosed with dechlor. Look on the forum as there is more in depth descriptions. Personally I would pull it out if possible and scrub with a toothbrush and dip it in h2o2.
If u can't remove it you can spot treat with excel or h2o2 using a syringe. Look up the safe dosing amounts of h2o2 if doing directly to the tank.