Insulating a block with neoprene is'nt done to 'soak up' the water, it's done to prevent air, which carries moisture, coming into contact with the cold surfaces of a TEC/Peltier~cooled/chilled~water waterblock. Moisture carried on the will condense and form droplets which will bugger your rig
(WW is'nt suitable IMO for TEC cooling)...
You don't need to worry about condensation if you use a 'straight' H2o solution of 'block, rad, and pump'...
What do you mean by 'slime'?, algae green natural growth?, or WaterWetter residue?...
I've not gotten any aldae slime in my rig yet, it's been running on tap water for six~nine months now, no 'clouding' either...