The gelatinous cube description reads:
Transparent (Ex): Gelatinous cubes are hard to see, even under ideal conditions, and it takes a DC 15 Spot check to notice one. Creatures who fail to notice a cube and walk into it are automatically engulfed.
Does this mean if you do not succeed on the DC 15 spot check the cube basically has greater invisibility as you cannot see it?
Or was this simply intended to be a spot check to notice it and not walk into it, and that it would be clearly visible once combat began?