Mold, or moss?
Moss was a constant problem when I lived in W. Washington. (It rains there a bit). Bleach will kill it. There's moss killer that lasts a little longer than bleach. Best bet is zinc strips.

http://bryophytes.science.oregonstate.edu/page24.htm

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past MB rides:
'68 220D
'68 220D(another one)
'67 230
'84 SD
Current rides:
'06 Lexus RX330
'93 Ford F-250
'96 Corvette
'99 Polaris 700 RMK sled
2011 Polaris Assault
'86 Yamaha TT350(good 'ol thumper)