SAN FRANCISCO—The head of California’s largest union was arrested on Friday, a day after the well-known activist was charged with grand theft and tax fraud. Alma Hernandez, the executive director of SEIU California, and her husband Jose Moscoso were charged on Thursday with allegedly underreporting some $1.4 million of income between the 2014 and 2018 tax years. Another complaint alleged that Hernandez, while working on a political action committee for a Democratic state senate candidate in 2014, approved a $11,700 payment to her husband for services he did not provide. Moscoso also allegedly did not disclose that his air duct-cleaning business had multiple employees, resulting in over $300,000 in unreported wages. Hernandez was being held at The Sacramento County Main Jail was and was ineligible for bail, according to the Sacramento County Sheriff’s Office’s website. SEIU California said that Hernandez, a 42-year old who had led the union for 11 …