Lot of people try to use the slang or short form on social interaction platforms and in day to day life as well. slangs/short forms help people to say or react quickly on daily basis.
There are lot of slang people use on Snapchat and Instagram app on daily basis.
Recently people are using the WTM on very frequent basis on Snapchat and Instagram.
If someone has replied as WTM on your snap on Snapchat, it means they are asking that What’s The Matter, is everything okay or is there something needs to talk. So you can use the WTM (What’s The Matter) to check people how they are.
As someone has posted any sad snap on snapchat, you can react as WTM and ask if there is something needs to discuss or talk.