Scrape out white
Smaller white details in watercolor are always a problem. They are often too small to just save by painting around. It is possible to use masking fluid and a dip pen or similar tool to mask small things, but it requires planning. Another way to create small white areas in watercolor is with the help of a sharp knife.
The knife you use should be very sharp, a dull knife does not work at all. It can work with a small box cutter. The problem with this type of knife is the pointed blade, it makes it possible to scratch out lines that are white but they do not allow you any grading. If you want the ability to grade your white scratched surface, you must use a scalpel with a rounded blade.
There are disposable scalpel with plastic handles that are cheap. There are also metal shafts with replaceable blades. The important thing is to use a curved blade, not a pointed one.
Scraping out small white details is the last thing you do in a painting, it is not possible to paint on top of the scratched surface. The paper is destroyed where you have scratched. When using a knife on the paper, it is much better to apply only a little pressure and repeat the movement several times. So avoid scraping hard, it is better to scrape many times with only light pressure.