VC-17 (as I may or may not remember correctly) is ~90% Copper oxide.

The green color is probably coming from 'verdigris' a copper acetate
compound which forms when copper or copper oxide is in contact with
Acetic Acid. ..... which depending on the acid content (pH) of the
water will turn greenish or brown/purplish.

Try to soaking the 'green' in alcohol (ethanol - booze) and if the
green comes off easily or dissolves ... its 'verdigris'.

Ive been using VC17 on my race boat for aeons .... and every damn year
it comes out of the water a slightly different color .... a large
sewage treatment plant dumps into the lake.

In article , Ernest
Scribbler wrote:

The person who sold me my boat said it had VC-17 on its bottom. I'm looking
at the Interlux website and none of the colors look like what I have, which
is kind of a muddy greenish color.