I'm guessing the OP has a post-2000 Mainship 34 Trawler. That model came with 3 Pompanette T2000 helm seats on the upper bridge.
That chair is know for discoloration on the armrests. While it looks like mold it probably isn't--I don't know of anyone who has successfully removed it. Best guess it is the result of a chemical interaction between the armrest material and.. suntan lotion? Who knows.
Other boats with the same seats suffer as well. See this Grady White thread
for example. There is a note there that Pompanette would sell glue-on replacement covers.
I just cleaned mine and sprayed them Valspar Plastic paint. I spray some touch up coats every few years. Still on my first can of paint.