What i have is...
A server running Windows 2016 Standard server with a Hyper-V VM running Debian 9.9.0.
A laptop, dual boot, Windows 10 pro and Slackware 14.2.

I would like to login to the Debian VM from Slackware for management purposes. I can do it from within Win 10, but would rather use Slack.

What software should I look into for this?

Note: this is a small home network.

Hi,
It won't be as fast as with Win10 to Win10, but you could use Remmina

If it's just for management purposes you can use SSH.
If you need a single X client (a GUI text editor, for example) you can forward the client over SSH to your X server on Slackware. Easier to set up than xdmcp.
mousepad will be forwarded to the X server on Slackware.

If you need the entire desktop then set up a VNC server on Debian. x11vnc allows you to connect to the real X server on Debian, the display as you see it when sitting at the Debian console. Other VNC servers create virtual X displays.

Since it's a home network you can forgo some of the security precautions you would otherwise take.

I use ssh for such things. If you need a full graphical desktop interface, a friend uses vnc and likes it.

At home and work I access several remote systems, many of which are virtual. Some are virtual machines and some are containers. For Linux systems with no X, I use SSH. For systems with X I use SSH and VNC. For VNC I use Remmina.

Important to access VMs in this way is to ensure the VM is accessible through the host. I am unfamiliar with Windows Hyper-V, but the common approach to provide access to VMs is to bridge the virtual network controllers. Once bridged, the VM will be on the same subnet and then any common access tool will work. As this is a home network then I'll presume everything is already on the same subnet.

At home and work I do not use SSH passwords. Only key pairs. On all of my client systems I passphrase protect my private keys. A somewhat nominal two factor authentication. I use ~/.ssh/config to create easy to remember aliases of all systems that I connect.

At work I am the person who supports SSH, even for the folks using Windows. We use PuTTY for Windows SSH access.