Hi guys, I just recently (Friday) acquired a new turtle (yellow bellied slider). This was unexpected, my boyfriend was trying to be nice, I recently had to give my cat away after becoming severely allergic to him and he remembered that I had turtles as a child and thought this would cheer me up. (I should note, my mom and I had a pond outside that we built for turtles that lived in the area, not the same thing as having an indoor turtle!) So I am still in the process of finishing setting everything up, he got most of the basics covered and I have the turtle (His name is Wood Polish) in a 40 gallon tank and he seems very happy. My neighbors moved out this weekend and my dad had mentioned to them that I had a turtle, apparently they had one as well that they did not want to take with them so they left it in a bowl on our doorstep this morning with a note asking me to take care of it or let it loose into the woods. The poor turtle would never survive out there so I decided to take him in, clean him up and feed him. According to my neighbor, the turtle is about 5 years old. He has some white gunk on his shell, not sure if he just needed to be cleaned or if it could be something more. I tried to take a good (non-blurry) picture of the shell.

Any help is appreciated, as this is all pretty new to me. Thanks in advance!
I'm trying to take more pictures of him since his shell looks like it could be in bad shape and will upload them when I get a decent photo.