mongod is the database process that BIMserver/cloud is built on. Seems like you've run into some bug. Don't have a solution, but for reference - our BIMcloud (v21 on Windows 10) server with 14 active projects and over 60 users only uses 24,8MB of memory.
The memory usage of the mongod.exe depends on the size of the BlobCache.
Send and receive all of the changes!
I recommend you to fully stop the BIM Server and then delete the BlobCache folder.
\Program Files\GRAPHISOFT\BIM Server 19\BIMcloud Server 19\BlobCache\
Delete the folder itself. It will be regenerated.
Besides that I recommend to update BIM Server to 9001.