I just looked at a picture of the rear panel of the HD24.
So, you have only a "Word In" and it says "75 Ohm". This means that IF you use a BNC cable for Word Clock sync, then the D8B has to be Master, and the HD24 has to be slave (b/c it only has a BNC Word Clock Input, but no Word Clock Output, that the D8B would need to sync to another device as slave).
In this scenario, you wouldn't have to use a terminator, though, since both, the D8B end, at Apogee Clock Card's BNC OUT, as well as the "Word In" BNC connector at the HD24, are terminated already.
If you'd rather not use a BNC word clock cable, then I don't know if the HD24 can sync to external devices via one of the ADAT/Lightpipe connectors or not. Most devices CAN do that, but some CANNOT (for example, the D8B). You need to check the manual for that.
If the HD24 manual says that it CANNOT sync to an external device via one of the ADAT/Lightpipe inputs, then the D8B MUST be Master and the HD24 MUST be slave - and you MUST use a BNC Word Clock cable from the Apogee Clock cards BNC OUT, to the HD24's BNC "Word In". Then set the HD24 to be "Word Clock Slave" or to "Sync to External" (however this is called on the HD24).
If the HD24 manual says that it CAN sync to an external device via one of the ADAT/Lightpipe inputs, and your D8B has the Apogee Clock Card (as you said), then you have a choice if the D8B or the HD24 should be Master (and the other Slave). You might still have to select which of the ADAT optical inputs on the HD24 would be the "Sync Source" or "Clock Source" in this case. It really depends on the device if something like that is auto-detected or must be set manually. So again, check the manual for the HD24 (I've never used one of those).
Sorry, I know Word Clock sync can be confusing if you haven't dealt with that before. But that's really important for getting audio to work properly, and without artifacts that happen in transfer, like pops and clicks. So, it's worth it to read up on what that is, and how it works a bit more
Best of luck!