Just like @jbkirk I mix mine very thick but there is almost always a bit of fluid that drips out into a different bucket that I set the strainer over and when it's time to feed, I pour the dripped liquid back into the mix. As jb said, if you keep the top covered with water, it will not mold.
I don't think you are feeding FF but rather wet food. That shouldn't harm them in any way but will give them additional fluid.
I haven't started a new batch in over 3 years but when my mix gets down to about 1/4 of material, I add a couple glugs of raw vinegar and more water, mix the whole mess up and immediately pour more dry crumbles into it. I don't even bother to stir the mess because it's so thick, it would take a machine to do it. I have learned by experience how much dry material to dump into the mix, rarely over-doing it. I watch it for a while and if I have added too much, I scoop some out into a smaller (2 gal.) bucket and cover it with water, to be added back after a couple uses.
If you don't understand any of this, just let me know and I'll try to make it more clear.
Edited to say, I'm wondering if what you are calling mold is what I call SCOBY?...A blueish film that covers the entire top of the mess?