Found it! The only nudibranch at CAS. Melibe leonina A.K.A. Hooded Nudibranch. This nudibranch swims by lateral flexions. The illustration I found below shows its movement in detail.
– Source: Illustrations of Melibe leonina at rest (top) and swimming (all ventral views). The sequence of drawings on the bottom show 50% of a swim cycle, as the nudibranch goes from being fully flexed to the right, to fully flexed to the left. Note the slight twisting of the posterior portion of the foot, which creates a sculling motion that propels animals in the ventral direction.http://www.journals.uchicago.edu/doi/full/10.2307/1543383