Hi @Hilko
Though there is not a specifics type for SharePoint Custom Form, but all the app display names are generated by the system, format: SPlistName on SPsiteName forms .
The workaround is to add a 'Filter Arrary' action to filter out SharePoint Custom Form Apps.
1. Add a 'Filter Arrary' action after 'Get Apps as Admin' action
2. set From field to value from 'Get Apps as Admin' action
3. Click advanced mode, enter the following codes:
@Not(And(contains(item()?['properties']?['displayName'], 'on'),contains(item()?['properties']?['displayName'], 'forms')))

Sik