Yes, mold can grow in your Hydro Flask. Mold is a non-toxic fungus that grows on the surface of food and other materials. Some molds are dangerous to people—they can make you sick if you eat them or breathe in spores. But most molds are harmless.
Mold can grow on almost any material, including your Hydro Flask! To be clear: the mold won’t hurt you or make your water taste funny (unless the bottle has been filled with something), so don’t worry about it!