SSH from Linux to Mac

 On the Mac: enable remote login

# sudo systemsetup -getremotelogin

If the result if "remote login is off", then turn it on:

# sudo systemsetup -setremotelogin on

Now the users on the system are able to log in. You have to log in via your user name. But what is it. On the Mac try this in a terminal:

# whoami
# arwen

Then you can read your username, and log on:

# ssh arwen@192.168.0.123

Based on this article.

On the linux system

Enable openssh server, see this article. It's from Arch Linux, but here the actual distro does not matter, since we work in the terminal. 

Concerning the systemsetup

Actually the systemsetup is a powerfull swiss knife for the system administrator. Read the man page.