Serama are from Malaysia, Japanese bantams are from, well, Japan.
Japanese have very short legs and a completely different stance than Serama, and also come in specific colors for the breed, along with having pure red combs and wattles. Serama have longer legs, stand very erect with their chests puffed out and the tail and wings almost perfectly vertical. There isn't any one defined color or pattern for the breed, nor any specifications as to lobe or leg color. Serama also aren't officially a recognized breed by the APA, either.