[codetrotter]

Possibly.

To me, sign and absolute value are often separate concerns.

The series 1, 2, 3, … is increasingly positive.

The series -1, -2, -3, … is increasingly negative.

But I understand, and accept, that others may think differently about numbers.

[MobiusHorizons]

I think you could probably say either decreasing or increasingly negative, but not decreasingly negative, because that would describe a sequence that starts from a negative number and moves towards zero (but hasn’t gotten there yet)

[noam_k]

I would argue that using the -ly suffix here is wrong:

The series 1, 2, 3 is increasing (and) positive.

The series -1, -2, -3 is decreasing negative.

The series -3, -2, -1 is increasing negative.

Edit: formatting