Do you have a steamer? It doesn’t have to be fancy, just a little cheap Bissell steamshot will do the trick. Hit this with some steam and scrub.

If not, cover it with very hot water and let it sit for a few minutes. Try to scrub and drain. If it needs more, cover it with hot water again and add a cleaner like soap or Oxyclean. Scrub and drain after like 10 minutes.

You could also try cleaning it with Comet or a paste made of baking soda and water.

Lastly, you may also consider using a drill brush if the other methods aren’t cutting it.