Is it possible to detach some specific database from MongoDB Server?
I tried to search the answer with google, but without success.


I'm pretty sure MongoDB does not offer "offlining" or "detaching" databases like traditional RDBMS products do, at this time (hence you not being able to find anything on it).

Have you set storage.wiredTiger.engineConfig.cacheSizeGB
 and hostInfo.system.memLimitMB to limit the RAM being used by MongoDB? If not set it will default to 50% of (RAM - 1 GB), which might be your actual problem.  

AFAIK Mongo shouldn't be loading unused database data into RAM, and the RAM to keep databases online should be negligible.