Mark,that looks like a skin fungus, but I've never seen it on the head like that. It could be loose skin that is shedding, but looks more like fungus. Does I come off with a brushing with a toothbrush?
If it does not come off, you need to treat. This fungus is like a skin rot. It will destroy tissue and actually eat away at the limbs and tail and can kill. It is normally from water quality and/or chemistry that is not compatible with that species turtle. Be sure the water quality is good. Check on the genus Pelusios and see what water chemistry they do best in. I do not keep them, but I would imagine they need hard water with possibly a lower ph. But check that out. that is the main thing that triggers this. Diamond back terrapins get it frequently when kept in water that is not brackish, for example.
The best way I have found to treat is to give the turtle a bath in water with betadine. Put enough in to make the water a dark tea color. Let the turtle soak in that for about 30 minutes. The water should be deep enough to totally cover the turtle. Then dry dock the turtle in a container like a plastic shoe box with a moist towel. Dry dock for several hours then place back in its tank. That should clear it up, especially if caught early. You will see quick improvement over a few days.
Thank you.Just once in the betadine.