Celery Soup extremely bland
Alliums

It doesn't matter if they are garlic, onions, leeks or shallots, but every online recipe I can see for croc-pot celery soup has at least one of them.

If you are trying to rescue a batch you have already made, I would suggest experimenting with adding onion power and/or garlic powder or puree, be sure to get powders, not salts though. Add a little at a time and give them time to  become integrated in the soup to adjust the flavour, the result won't be instant, the powder needs to rehydrate before it fully releases its flavour.

The other element that I belatedly note is largely absent is fat. Fat carries flavours and adds satisfying mouth-feel to dishes. Again, most recipes I'm seeing online have more fat than the modest amount you will be getting from the milk. A couple of glugs of olive oil or a couple of table-spoons of butter will help bring out a richer flavour. https://www.thespruceeats.com/functions-of-fat-in-food-1328452