This sounds like blooming, which can occur naturally in rubber products. This can easily be treated with household items, using either oil or dishwashing detergent.
Oil Method
You will need;
Olive or Coconut oil about a teaspoon per boot & cloth or paper towel.
Directions;
Apply a teaspoon of oil to a clean cloth or paper towel. Start from the top of the boot and work down, a section at a time, wiping the cloth over the boots in a circular motion. Take care to avoid the soles of the boots, and give extra attention to areas with more blooming.
Continue this until the oil is absorbed by the rubber boot. Leave the boots to dry completely, and the blooming should have gone. If there are any remaining spots, repeat the process.
Dishwashing detergent method
You will need;
Clean cloth, about a teaspoon of dishwashing detergent, about 1litre of water & a spray bottle and a scrubbing brush can be useful – but aren’t essential.
Directions;
Mix the detergent & water and coat the outer of the boots. A spray bottle works well, or a cloth. Leave the solution on the boots for about 3-5 mins. Use a cloth to gently rub the detergent mix into the boots using a circular motion. Allow the boots to dry completely. If there is any remaining blooming, repeat the process.