Complex sentence: The colour of this fish is also variable, usually resembling the substrate, which makes it well-camouflaged when stationary but less so when it moves about, at which time it engages in characteristic short bursts of swimming activity.
Split into simpler sentences: The colour of this fish is also variable and usually resembles the substrate, which makes it well-camouflaged when stationary. It is more visible when it moves about, at which time it engages in characteristic short bursts of swimming activity.