Many thanks to all of you! Some conclusions:
I'll try vinegar, if not -Barnacle Buster and then - ultrasonic cleaner. How it works - I'll update in Spring.
To protect in future -McLube Sailkote dry lubricant looks very promising.
This problem happened the first time for the last 10 years on the same boat, same mooring. Probably I was too lazy and put only one layer of ablative paint. For boaters, who do not know how it works, quote:
"ablative paints are an older type of paint designed to create a hull coating which ablates (wears off) slowly, exposing a fresh layer of biocides".
After some analysis, I say no to Ultrasonic Antifouling Kit.
Such devices make to vibrate the whole boat including the
mast, shrouds winches ... The signal (let's say 20kH) consists of many harmonics (different frequencies) which may "find" a thing in a boat to get resonance with. If
gelcoat has some very small, not noticeable
blisters, they will grow or could be created. Also, the threshold of human hearing - 20kH, you might not hear it, but you feel it, and your
mast will be working as a speaker. Also, the mast could be similar to a giant dental ultrasonic plaque remover. By the way, dental
research:
"The results of this study suggest that ultrasonic scaling can cause further damage to teeth with enamel cracks..."
And, finally, I don't think
dolphins would like such
music.