Windows Defender finding Trojan in After Effects Media Cache?

Probably just what's known as a hash collision, i.e. the calculated checksum of your files having the same result as one of Defenders signatures for a malicious file. Indeed nothing to worry about. As suggested, simply exclude folders that produce such temp files from monitoring.

Mylenium

That's not something I've seen before, but can you just exclude your entire Media Cache folder from Windows Defender? I can't say that it will improve performance, but as AE is constantly creating cache files, those are ones you don't need to be scanned. Here's a link on how to do that, but note that "Windows Defender Security Center" is now just called "Windows Security." The rest of the article still seems to have current labels. https://www.windowscentral.com/how-exclude-files-and-folders-windows-defender-antivirus-scans