That's a tough one, especially while it is still this small. You may have to wait a bit to see what growth form it takes to really get the right ID. But...
If it were a Pavona, it's likely a pavona varians. There is also a good chance that it's a merulina... however, it's tough to say right now as both the pavona and merulina start as encrusting and then develop their characteristic shapes later on. I don't think it's a Pachyseris Sp..
Do you have a rough geographical location?
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