Aeolidiella glaucais broader than A. alderi when both are at full extension. The tips of the propodial tentacles (1) of A. glauca (and A. sanguinea) are often turned forwards, as in this image, but rarely, if ever, on A. alderi. Scale adjusted so comparison is of proportions on equally long images.
Actual lengths: upper 8.6 mm, lower 20 mm. Both from Menai Strait Wales.