So this is citrus scale, common on citrus and hard to really get rid of. More details are here.
I recommend removing every leaf on the plant then rubbing the stems with soap and water at five day intervals. You can use a rag or paper towel. Repeat at least three times and don't be afraid to do more applications
This is effective because:
- you are reducing the surface to cover by removing the leaves
- by using a rag or paper towel you are crushing the scale and ensuring complete coverage
- by repeating the soap and water at least three time you are getting the ones you didn't get the first time and the eggs that have hatched
Reduce water after removing the leaves until new ones grow. No fertilizer until it is clean.