Hello @Adriano,

If it is the app icon which is visible in home screen (Enterprise Home)
Please check into its menu.
Here is an example of Invoicing menu:

Check example in comment below.

-> web_icon="account,static/description/icon.png
The above line adds icon to the app menu where `account` is the module name and `static/description/icon.png` is the path of icon inside module.

If it is regarding the app icon which is in apps list
please check in description folder there you will have to add (if does not exist) a file named icon.png (icon for app).

Do the above and upgrade app list and then the module if required.

Hope this helps.
Thanks & Regards,
Email: odoo@aktivsoftware.com
Skype: kalpeshmaheshwari