The National Transparency Authority (NAA) / AFCOS, established by Law 4622/2019, is a single and independent authority that has the national coordination for fraud issues in Greece.
As part of its mandate, the authority acts as the national complaint reception point for co-financed programs / projects. You can see more about the role and activities of the National Transparency Authority on the website aead.gr.
To submit a complaint you can use the special EAD form, at the EAD address.