Q: Corrosion and battery acid on negative terminal connector.

I paid a good chunk of money to fix several things wrong to get my check engine light off on my '06 VW Jetta about 2 months ago. Some time later it came back on. It turned off and came back on occasionally, but lately it's been staying illuminated. I've taken it to AutoZone twice now for a diagnostic on it and their readers never finds an error code. The worker told me to try resetting it by taking off the negative terminal from the battery for a bit and then reconnecting it. As i was fixing to do so, I notcied there was a lot of corrosion and what looked like battery acid on the connector so i didn't want to mess with it. Could this have anything to do with the check engine light or why his reader isn't able to pick anything up? Thank you in advance for a response.