It may help if you can describe it in a little more detail. From the photo, it appears to be white, roundish, about 3mm across, and on, rather than under, the skin. Is that correct? Does the texture seem hard or fuzzy? Does it have a symmetric shape, which might indicate it's a flatworm? Did it appear suddenly at full size or did it grow? Do you see any redness around the bump indicating irritation or infection of the skin?
If you think it might be external to the skin, I suggest a freshwater dip which might knock it off.