That's pretty common with light colored bettas. My cambodian female is named Skelly (short for skeleton haha) because when she was super tiny you could literally see EVERYTHING inside her. Now that she's older it's better but if the light hits her a certain way you can still get an "x-ray view".
Lolly the Bully (banished from the sorority), Vacuum, Finnie, Princess Buttercup, Tiger, and Boomer (all girls); Captain America (our token male)
Dog. Children, and Husband
Frank, Sunshine, Peanut, and Mr. Muscles
Looks like a cellophane to me - they're usually see through like that. Its very cool though! I saw a halfmoon cellophane a couple weeks ago at walmart, and wanted so badly to take him home. But no matter how pretty the bettas are there, I won't buy one and encourage them to keep up with that horrendous care.