Can I incrementally back up my High Sierra SSD System (APFS formatted)
on my external HDD which is HFS+ formatted?

Thanks for answering in advance.

Excellent question.

And the answer is a, resounding, "absolutely!"

In fact, you can use almost any mix of filesystems.

Unlike backup solutions that simply copy files, QRecall stores the information about your captured files in a database. So it doesn't matter what the capabilities of the destination filesystem are, as long as it can hold an archive.

This makes QRecall unique in its ability to capture items from a filesystem that supports user permissions, hard links, extended attributes, access control lists, and compression, and faithfully preserve them in an archive on a volume that doesn't support any of those features.