White/grey thin oval painful spot under tongue, gone in a week, surrounded by red area, faded away, didn't bust/pop, think was canker sore, safe to kiss?

Trauma?: It could just be an injury or burn (these are often caused by Aspirin left in contact with oral mucosa for too long). If this has been a single isolated incident, that is most likely. Oral herpes viral lesions which are contagious through kissing, usually take 2 weeks to heal and are often on the lips but can be on the tongue as well. Safest bet is to defer until fully healed...

If it is gone: ok to kiss. May be a canker sore, burn, trauma , herpes,.... if it reocurrs have it checked out.

It is safe: If no lesion is present, it is safe to kiss.