That looks to me like a pop-out turkey thermometer, yes; it's basically a spring-loaded plunger that's stuck to itself with a food-safe adhesive that, at a certain temperature, unsticks, which means the spring built into it can 'pop out' the little button to give the cook an indication that the turkey breast has reached a safe internal temperature.
However, I would nonetheless recommend that you remove it, for the following reasons:
- These thermometers are typically calibrated for use in an oven, with the turkey being roasted; the different cooking environment of a slow cooker might effect how the heat spreads through the bird and make the thermometer's reading less useful.
- The temperature these thermometers are set to pop at, even under ideal conditions in an oven for roasting, is often significantly higher than even the FDA standard temperature for safe poultry and generally results in what most would consider a dry, overcooked bird (at least for the breast meat). The more advanced wisdom is to bring your turkey to a lower internal temperature - around 160-165F or 71-74C, check using a meat thermometer, ideally an instant-read one - and make sure it stays there a little while, generally by resting the meat after you take it out of the oven.
- If you can get it out easily without mangling the breast before cooking, it's one less thing to have to remember later; if it's stuck in there, though, it might be easier to take out later once the meat is cooked.