Growing, Phyllidiella pustulosa changes in appearance. It has following characteristics :
- the black body with pink tubercles
- the tubercles are arranged in clusters, amalgamated in juveniles and separated in adults
- the mantle is bordered with pale pink
- the rhinophores are triangular black

Phyllidiella pustulosa can be mistaken for Phyllidiella granulata. Here are some characteristics to distinguish these species :
- P. granulata is greyish and it has 3 groups of tubercles in the centre of the mantle
- the tubercles are more acute in P. granulata than in P. pustulosa

Also, Phyllidiella pustulosa can be mistaken for Phyllidiella zeylanica. But in P. zeylanica tubercles form ridges rather than clusters.

Furthermore, Phyllidiella pustulosa looks like Phyllidiella annulata. Here is how to distinguish these species :
- in P. annulata the mantle isn't edged with pink like in P. pustulosa
- in P. annulata the tubercles form rings rather than clusters

Red Sea