What is a Slider?
The slider or back spinner is the reverse of the top-spinner. Instead of bouncing and kicking as the top-spinner does, the back spinner delivery will skid onto the batsman. (Sources:1, 2)
This article from cricket.com.au will help in understanding slider. Youtube
What is off the seam bowling?
Seam is the raised stitching around the circumference of the ball. While seam bowling is, using the seam of the ball to cause the ball to bounce in an unpredictable fashion when it hits the pitch.
Off the seam term is used by commentators to describe the ball that bounce unevenly or deviates from the length of the bowling (movement of the ball), after seam of the ball strikes the pitch.Need citation
Sources:1, 2, 3