I am still not sure how to tell the difference between mille and prostrata. I have read that they grow differently but wouldn't that depend on flow and lighting? Yours looks like a standard blue millepora to me but that my just be the photo.
For true coral id, I think a biologist has to look at it under a microscope, therefore, we as hobbyists tend to throw these terms around by what we observe visually. Not true scientific identification processes.