Is it possible to search for emoji usage in Teams?

Depends on what you mean exactly. Both emojis within messages and reactions to messages are stored for compliance needs, but searching/reporting on them involves different methods. For reactions, you can do it directly via the audit log, see Tony's article: https://office365itpros.com/2025/11/07/teams-reactions-emojis/ 

For emojis within messages, you need eDiscovery/content search. You will need the "identifier" for the emoji, such as "grinningfacewithsmilingeyes", or the full path, if it's an emoji from external source. Doable, but kinda annoying.