As you may know by now, octopuses can control both their color (thanks to their chromatophores) and their skin structure. The size of their skin projections, called Papillae is controlled through muscular hydrostat.
A muscular hydrostat is a biological structure mainly made of muscles with little or no skeletal support, which can easily change shape to bend, extend, etc. Some other examples of muscular hydrostats are our tongue and an elephant’s trunk. This is how the octopus can appear smooth, bumpy or spiky at will.