Unlike other water and sports drink receptacles, running packs and hydration bladders pose a unique challenge because you can't simply unscrew the lid and go to town with a sponge or scrubber. Runners, especially distance runners, can greatly benefit from having a hydration pack! They really help step up your training by keeping you hydrated when you are ready to venture out into some seriously longer miles. For marathoners and ultra marathoners, having a hydration pack or some form of accessible water is absolutely necessary, to avoid potentially threatening dehydration scenarios. But they certainly are tricky to keep clean.
One way to get out the bacteria and the slimy stuff you are seeing is to fill the pack with warm water and a bacteria killing disinfectant dish soap. You can even throw in some heavy duty bleach, as long as you make sure you thoroughly rinse it out (definitely do not want to be ingesting that stuff!). Shake it all around so that the entire inside of the pack is coated in soapy detergent water. Alternatively, you can use common products typically found in your pantry (such as baking soda or distilled vinegar) and mix them with the water to kill germs and clean the pack. Let this solution sit for about half an hour after you shake it and before you dump it out. And then to avoid bacteria forming again because the pack does not dry quick enough, make sure it is inflated and there is air inside of it so that the insides of the plastic do not stick together. Let the pack air dry completely.
To ensure that bacteria does not grow in your pack in between long runs when you are storing it and not using it, you can take precautionary measures by washing it out using the above process every time. You can also store it in the freezer, where temperatures are low enough that bacteria will not grow. (A bonus to this is that you will have a cold pack when you are ready to run, that will help keep your water cooler and more thirst-quenching for longer!).