Never seen a circle of slime before any ideas???

Found this almost perfect circle is what looks like slime on a tree on my way from work today I have never seen this before so does anyone have any idea as to what it is or could be. I know it's not the best/clearest photo but I did try honestly.
This really is the best I could do today for a blip as I have not been in the best form,,, as usual my back pain, back to work, my MRI results from yesterday didn't help either knowing I already had 2 lesions on my brain but in 6mths I now have a 3rd one and something else I don't understand they also found.
So in a nutshell today has not been a good day at all but as I always say tomorrow is another day.