How do I grab the label of a LJ Dynamic dropdown fields
The LJ Dynamic fieldtype only stores the value as a string, or as an array of values for the multiselect fieldtypes. The label in the settings is there just for displaying in the admin.

Either you could use the store.title instead of store.storeId as the value, if you only need the store name in your template. Or you could use the store id to get the data from the store in your template. Something like:

{% set store = craft.entries.section('stores').storeId(entry.stores).first() %}
{{ store.title }}