Was anything leaking on the serpentine belt?
How many miles on your Tensioner Aluminum Pulley?
Did you modify the Tensioner when you installed the Pulley?
I checked three Belt Tensioner Aluminum Pulley Installations and all is well.
One Tensioner pulley with 6,000 miles. I removed the 6,000 mile Tensioner Aluminum Pulley and checked the bearing which was found had no play and very smooth when rotated.
What is that all about?
Belts and oil do not mix
The problem of an oily belt would be between the belt and the Harmonic Balancer driving the belt and Alternator Pulley, Power Steering Pulley, Water Pump Pulley, AC Pulley getting power from the belt.
Check the Bearing and what is meaning of oily residue?
Even if the belt was oily the Tensioner Aluminum Pulley is free wheeling so I would expect no issues. Spin the Tensioner Aluminum Pulley before you remove the Tensioner to see if any play, resistance, interference.