if the simple green left it looking dry, then the finish on it was just an oil, which most astringent cleaners will suck out ... easy enough to re-oil, but it's the oil that's feeding the stuff so you may have to sand it to get rid of all the stain .... you'll avoid the fungal problems if you put a hard finish on it, urethane as you say - I've had good luck with 'Minwax Helmsman Spar Urethane' on outdoor wood
Bill
