Hi there @RachelMariposa!
Don here from Shopify.
Thanks for sharing your query with us here in the Shopify Community.
In cases where third-party apps add or require you to add some code to your theme's files for the use of that app, this code will not automatically be removed along with the deletion of that app from your store.
Not to worry, as there are a couple of options available to you to get that removed.
If you remember which .liquid files were edited for this app and have not made other changes to that file since then, you may be able to revert the code changes to a previous version of that file.
If this doesn't work for you for any reason then you can absolutely contact the developers of that app who would be responsible for providing support for their creation including helping you to remove that leftover code.
I had a look for a Showcase - Shop Instagram app in our app store and I think this is likely the correct one.
You can use the contact details listed there to get in touch with the app's developers who'll be able to get that code removed for you.
Are you looking for an alternative app for some Instagram functionality or did you decide against this for your business?
Let me know if there's anything else you need to work on next for your store.