Agreed, looks like Wet Spot to me. Bacillus spp more than likely. I don't see a sliver of mycelium to transfer off, Bad NEWS...to the trash can receptacle she goes. Keep at it, learning how to not fumble and maneuver in a SAB is key with plates. It's the main reason I wish I had a FH. It IS a bit cumbersome doing plates in a SAB... BUT can and has been done time and time again. We all experience loss of plates, just part of it playing with a nutritious media and a real touchy procedure. Sanitation is the key since we are manipulating the air for a certain period of time to get our work done. Always remember, people who preach they don't experience ANY contamination are either lying (EGO) or they've been extremely lucky and haven't been doing this long enough. I personally don't lose jars often, but plates... you WILL... only a matter of time. Sorry for your loss, nothing like getting back in the batters box and taking another SWING. All the best to you and your projects.