It is unclear to me how cloudControl MySQLd addon works. My understanding of MySQLd is that it is a MySQL server that can/will work with unlimited apps.
But since all addons are only app based, this could also mean that I cannot use the same MySQLd server on multiple apps.
Could anyone please help me understand if one MySQLd instance can be used with multiple apps hosted on cloudControl?
There are two concepts on the cloudControl PaaS. Applications and deployments. An application is basically just grouping developers and deployments together. Each deployment is a distinct running version of the app from a branch matching the deployment name. More details on this can be found in the Apps, Users and Deployments documentation.
All add-ons are always per deployment. We do this because this way we can provide all credentials as part of the runtime environment. This means you don't have to have credentials in version controlled files. Thich is a huge benefit when merging between branches, because you don't risk accidentally talking to e.g. the live database from a dev deployment. Also add-on credentials can change at any time at the add-on providers discretion.
For this reason separation between deployments makes a lot of sense. Usually your dev deployments also don't need the same database power as the production deployment for example. So you can easily use a smaller plan or even a shared database (e.g. MySQLs) for development. You can read more about how to use this feature inside your code in the Add-on documentation.
Also as explained earlier, add-on credentials are always provided as part of the runtime environment. Now credetials can change at any time at the add-on providers discretion. These changes are automatically provided in the environment and the app processes restarted. If you had hard coded the credentials as would be required for the second app, this would mean the app would probably experience downtime.
Last but not least, it's usually very bad practice to connect to the same database from two different code bases in different repositories, which would be the reason to have a second app. This causes all kinds of potential conflicts and dependencies that make code changes and database migrations extremely hard to maintain over time. The recommended way would be to have the data owned by one code base only and provide an API to access that data from the second code base.
All this being said, it is technically possible to connect multiple deployments or even apps to the same add-on (database or anything else) but highly advised against.
If you have a good reason to connect two apps/deployments to the same database I would suggest you manually launch an RDS instance at Amazon (MySQLd is based on RDS) and provide credentials for that through the custom config add-on to both of your apps/deployments.
I hope this answers your question and also explains the reasons.