We always go through the message many times \”Magento 2 indexing – One or more indexers are invalid. Make sure your Magento cron job is running\” during admin changes specially during catalog changes.

Simply solution is that we can use Magento 2 Reindex command

php bin/magento indexer:reindex

in case of you find memory limit issue you can check check with below command.

php bin/magento -dmemory_limit=2G magento indexer:reindex

Thank You.!