add_action( 'init', 'projectslug_register_meta' );

function projectslug_register_meta() {
	register_meta(
		'post',
		'projectslug_mood',
		array(
			'show_in_rest'      => true,
			'single'            => true,
			'type'              => 'string',
			'sanitize_callback' => 'wp_strip_all_tags'
		)
	);
}

register_meta(
  string $object_type,
  string $meta_key,
  array $args,
  string|array $deprecated = null ): bool

$object_type
Type of object metadata is for. Accepts 'post', 'comment', 'term', 'user'

$meta_key
Meta key to register

$args
Data used to describe the meta key when registered.

<!-- wp:paragraph {
	"metadata":{
		"bindings":{
			"content":{
				"source":"core/post-meta",
				"args":{
					"key":"projectslug_mood"
				}
			}
		}
	}
} -->
<p></p>
<!-- /wp:paragraph -->

<!-- wp:group {"style":{"elements":{"link":{"color":{"text":"var:preset|color|primary"}}},"spacing":{"padding":{"top":"var:preset|spacing|30","bottom":"var:preset|spacing|30","left":"var:preset|spacing|40","right":"var:preset|spacing|40"}},"border":{"top":{"color":"var:preset|color|primary","width":"2px"},"right":{},"bottom":{"color":"var:preset|color|primary","width":"2px"},"left":{}}},"backgroundColor":"pale-gray","textColor":"primary","layout":{"type":"constrained"}} -->
<div class="wp-block-group has-primary-color has-pale-gray-background-color has-text-color has-background has-link-color" style="border-top-color:var(--wp--preset--color--primary);border-top-width:2px;border-bottom-color:var(--wp--preset--color--primary);border-bottom-width:2px;padding-top:var(--wp--preset--spacing--30);padding-right:var(--wp--preset--spacing--40);padding-bottom:var(--wp--preset--spacing--30);padding-left:var(--wp--preset--spacing--40)"><!-- wp:paragraph {
	"metadata":{
		"bindings":{
			"content":{
				"source":"core/post-meta",
				"args":{
					"key":"projectslug_mood"
				}
			}
		}
	}
} -->
<p></p>
<!-- /wp:paragraph --></div>
<!-- /wp:group -->