I would say it is just because it's such a common coral why would anyone sell a fake. I think once it settles in and colors up you will see it look like it should.
I think it is not a yonguei by the way it is growing...but I am not an acropora expert...so it is only my opinion...
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Based on what I can see it sure looks like it. Mine has some slight pink areas on the new growth areas which are a much lighter almost white than the normal bright green. Yours looks very pale which would explain the white polyps. The polyps are the same color as the rest of the coral usually so if yours is partially bleached that's why they are white.
I can't really give you a correct ID, esp at this size of frag and only a side shot, but I have my doubts as it being a Acropora yongei, not enough to 100% rule it out
(I know it doesn't look like your typical green slimer, but Acropora can vary big time in in over all look and structure)
the radials do have the tubular structure like A. yongei but they look like they lack the dished openings (need top down) cant really tell from a side shot how they are structured very well, I know I say this alot but top downs really help to determine axial corallite opening size, wall thickness, also get a better look at the radial arrangement and openings as well.
Just my .02
I think it is not a yonguei by the way it is growing...but I am not an acropora expert...so it is only my opinion...
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