Stylus stuck in groove.

@rvpiano I assumed you already had the solution since you have an ultrasonic cleaner. You can also just use distilled water if that is more handy. If you do decide to use solution, you can try adding a few drops on the area and letting it soak for a min or two before wiping.

I’ve also found it pays to be persistent. Keep wiping a while til you see the gunk gone (if it is visible). With a microfiber cloth, you don’t have to worry too much about scratching even with a good amount of pressure applied. 

The Manual Method Using The Link, has transformed my LP's, even the very worst that are from my Teenage Years are rejuvenated and totally playable.  

PACVR-3rd-Edition

@rvpiano I've occasionally come across a record that skips. Each time it was a damaged record (scratch). Can you see the troubled area under a magnifying glass? It might not be dirt.

I have successfully used small alcohol wipes while the LP is on the platter, then a wipe with an eyeglass cloth: but some like you describe are stubborn.

Stop being a Sissy, scrub the bejesus out of it manually. (baby scalp brush).

My mix: start with ’record cleaner’ liquid; strengthen it with alcohol; slime it with a few drops JetDry dishwasher spotless stuff; the spinner is made for cleaning, but I use it only for rinse with distilled water, then a light touch of cloth to remove excess water, then into the rack to dry.

I keep the paper label dry using a lid from a soup container.

@pindac   +1.

Written by a high class expert in cleaning, who's generous to boot. We're lucky to have access to that level of expertise.