The FREEDOM+ Surf electrodes are made from the highest marine grade stainless steel known as 316L, there is no other known material more resistance to salt water and electrolysis than 316L.
The brown staining or "tea staining" is a result of electrolysis, there is no way to prevent this occurring and it will occur during each surf session. This tea staining has been occurring on all electrodes over the last twenty years in the Ocean Guardian range, although more noticeable with the FREEDOM+ because the stainless steel is more visible.
This staining does not affect the performance, however you must clean the stainless steel after every surf to prevent this damaging the plastic surround, ideally clean it with Sodium bicarbonate, this is a cheap non chemical based domestic cleaning product. Simply create a paste and using a soft sponge, rub the electrode surface and you'll find the tea stain will come off relatively easily.
Do not use any abrasive scrubbers or chemicals on the decal antenna as you will damage it.