'Safe' in British and multicultural London English is a casual greeting and acknowledgement, equivalent to 'cool', 'thanks' or 'all good'. 'You good?' — 'Yeah, safe.' If your teen ends a message with just 'safe', they are signing off politely. It can also be a compliment ('he's safe' = 'he's a good guy / trustworthy'). Friendly and harmless, common in British youth speech since the 1990s.