Why do I have a random string of letters associated with my customer list on Shopify?

Those are typically bot accounts. These random strings of letters are the best way to differentiate bot emails from emails of real people.

One of the solutions to your problem could be to sign up for an app called 'Shop Protector' - https://apps.shopify.com/ellipsis-human-presence-technology

It blocks most of the bots and saves you from being inundated, random emails being added and your customers being spammed.