Named after the Alhambra
It's called Spanish because its interior copies the Moorish style of the Alhambra in Granada, fashionable for synagogues in the 19th century, not because Spanish Jews founded it.
The newest synagogue in Prague's Jewish Quarter stands on the site of the oldest. Its name has nothing to do with Spain's Jews, and everything to do with its decoration. Here's what you're looking at.

It's called Spanish because its interior copies the Moorish style of the Alhambra in Granada, fashionable for synagogues in the 19th century, not because Spanish Jews founded it.
It replaced the Old Shul, the oldest Jewish house of prayer in Prague, and was built for the Reform congregation, with an organ, as Reform synagogues had.
František Škroup, who wrote the Czech national anthem, was organist here in the 1830s, at the earlier synagogue on this site.
Because of its Moorish Revival decoration, modelled on the Alhambra in Spain.
In 1868; the interior decoration was completed in 1893.
Yes, with a Jewish Museum in Prague ticket; it also hosts concerts.