What it might be

With any mirrorless camera that does no close the shutter on lens changes, you will end up having dirt on your sensor occasionally. While I cannot say, what kind of dirt you have there, I would guess it to be a little piece of skin flake as it seems to be translucent.

Tips for preventing dust from getting on sensor

1. Hold your cam and lens mount downward, when changing lenses, so that any dirt rather falls out of the cam

2. Check your sensor periodically for dust with an illuminated magnifier.

3. Always keep either a lens or lens cap on the camera body.

Tips for Cleaning

1. If you see something, it is almost always too late for the in-camera vibration method. Then remove lens, hold mount downwards and use an air blower.

2. If that does not help, try a lens brush, then blower.

3. If that does not work, use a speck grabber (a kind of stick with some sticky gel on one side). With that you just collect the speck and then transfer it to some sticky material which came with the stick. Note: Some people do not trust these, I succeeded in keeping my sensor clean until now using them.

4. If that does not work, you need to use a moist sensor swab on it. Note: The sensor itself is under a layer of glass, so you cannot damage anything unless you are really reckless or use way too much cleaning fluid.