To remove the tag "Get your own free account”:
Add this in /path/to/nextcloud/config/config.php: 'simpleSignUpLink.shown' => false
https://help.nextcloud.com/t/remove-link-get-your-own-free-account/41131/5
To remove the tag "Nextcloud" slogan:
Edit /usr/local/www/apache24/data/nextcloud/lib/private/legacy/OC_Defaults.php:
$this->defaultEntity = '<some_entity>'; /* e.g. company name, used for footers and copyright notices */
$this->defaultName = '<some_name>'; /* short name, used when referring to the software */
$this->defaultTitle = '<some_title>'; /* can be a longer name, for titles */
$this->defaultBaseUrl = '<the_base_URL_of_the_site';
and
$this->defaultSlogan = $l10n->t('private storage');