HONG KONG—Hong Kong police arrested the former executive editor-in-chief of the now-closed pro-democracy Apple Daily tabloid on Wednesday, local media reported, the latest arrest in a national security investigation of media tycoon Jimmy Lai’s newspaper. Police, who typically do not identify individuals under investigation, said in a statement they had arrested a 51-year-old former male newspaper editor on suspicion of “conspiring to collude with foreign countries or foreign forces to endanger national security.” Media group Next Digital, which published Apple Daily, did not immediately respond to a request for comment. The popular tabloid of staunch Beijing critic Lai folded after several hundred police raided its headquarters on June 17 and froze key assets and bank accounts. It printed its last edition on June 24. Police have said dozens of the paper’s articles may have violated the city’s China-imposed national security law, the first instance of authorities taking aim at media …