You’re other plugin, whatever that is, is storing a serialized array. The code that I posted above is a WordPress function that will unserialize a value into an array if it is a serialized value.
Beyond that I don’t think I quite understand what it is you’re trying to do.
You’re trying to get this value stored by another plugin, but what are you trying to do with it? Are you trying to allow a front end ACF form to update that value so that the other plugin can find it and use the updated value?
