I can find it in system monitor.

but, [systemctl list-unit-files | grep  mysql ] did not show anything.

After googled, I found it belong to akonadi which seems to be a component of KDE.

use following to kill the process

pkill -f akonadi

Then

edit ~/.config/akonadi/akonadiserverrc

change to following

StartServer=false

reference:

https://askubuntu.com/questions/762062/akonadi-services-and-mysqld-use-too-much-memory-in-kubuntu-16-04