Monica CRM is a great open source contact manager, but as of early 2019, there's no official mobile client.
Adding contacts via the web interface can be a little slow on mobile. When you're out, you might not have fast/reliable internet.
When you add someone new, you probably want to add a few notes. In the web UI, this can be done only after you've submitted a form with the person's name.
This app is very simple: it lets you enter a person's name (forename and surname) and some notes. When you're done, it will immediately let you continue adding more people.
There's no waiting, because the new contact and new notes are uploaded in the background. And if you have a bad network connection (or are on a plane!), the information will be saved on your phone temporarily. Once you have an internet connection, it's just one tap to upload all the pending contacts to Monica.
In order to use the app, you need a 'Personal access token', which you can create in the Monica web app. Just click the settings icon in the menu bar, then click 'API' on the left hand menu. You will see a button 'Create New Token'.
Click that button, give the token a name (doesn't matter what it is) and click 'Create'. Then copy that token and paste it into the 'Settings' screen in this mobile app.
None of your contact data is sent anywhere except the Monica CRM server you specify in the app's settings. All communication is directly between your mobile device and that server.
This app has no relationship with Monica CRM or its creators. If the app works for you, great! If not, please don't bother Monica's developers for support, as they likely can't help.