Addresses under greybull.site (for example mysite-acme.greybull.site) are websites published by customers of Greybull, a Dutch hosting provider. Greybull hosts them; their content is the customer's.
To report phishing, malware or other abuse on a greybull.site address, email abuse@greybull.cloud with the full address and what you found.
Security issues with Greybull itself: security@greybull.cloud.