The white "mold" could simply be natural sponge growth. A close up detailed pictured would help to ID
As for the oil slick, have you put in an overflow system to a sump? Otherwise, the only way to prevent it would be to have some sort of surface skimming filter or protein skimmer. I have it on all of my tanks, with the right amount of surface agitation by pointing a powerhead at the surface, it does not hinder gas exchange. The only other thing i have found to work is using a HOB
filter, but IMO
it is not worth the addition problems it creates with salt creep, etc.