iMuto 20000mAh Portable Charger How do I charge this?

The short answer is no. Connecting a charger that supplies less power than this is rated for is extremely unlikely to damage the device as long as the voltage is the same (USB makes this easy because there are standards). A rating is just that. It's rated for UP TO a certain amount. If it's less, it will use less and charge more slowly. No harm done.

The Amazon listing for this Power Bank indicates it accepts 18 Watt input through USB C or Micro USB.

Let me do some simplifying of the math and some basics about electricity as related to charging just so you get a more complete picture.

Watts are a measure of "power" and is calculated by multiplying the Voltage by the Amperage. Voltage is a measure of energy potential and Amperage is a measure of volume. You can use this method to calculate the wattage of any charger if you check the numbers on it (it's often this information be listed) These two values are very interconnected.

Voltage is something that is a more "constant" number—a charger that's rated for 5V should always output approximately 5V. But amperage will vary depending on what it's connected to. Some devices do not need as much energy to work as others. Amperage is usually referred to as being "drawn." Even though that's a bit of an oversimplification, a circuit that only needs 1A should only draw 1A, even if more is available.

So the longer answer is also no. But to elaborate more, the voltage requirement of the circuit is met and as long as there is enough current available to power the components of the circuit AND pass through to charge the battery you're in the clear. Even a charger that's rated for a higher amperage (but the same voltage) won't typically harm the components, but it does generate more heat which can wear batteries out faster.

TL:DR. Don't worry. You're all good. It might actually be better for the device to charge at a lower current, but you might be waiting around longer for the battery to fill.