Members of this group can be notoriously difficult to identify. In my more pessimistic moments of trying to figure out an animal that seems intermediate between 2 or more species, I sometimes want to just call everything in the family a single species with a lot of color forms. I have changed my mind about the names of some of these species several times and will probably do it again.
The phyllidiids, like their somewhat distant relatives the dendrodorids, lack a radula, the "tongue" of hard teeth found in most other sea slugs and used for scraping their food into their mouths. Phyllidiids and dendrodorids feed essentially by spitting digestive enzymes onto their sponge prey, then sucking up the partially digested softened sponge tissue. Most phyllidiid species are highly toxic, with their toxicity generally derived from whatever sponges they are eating. These nudibranchs give off quite noxious chemicals when disturbed. Many years ago, I made the mistake of trying to keep a specimen of Phyllidia varicosa in a home aquarium in Hawaii. Sometime during the day, when I was away from home, the nudibranch, possibly bothered by a fish or stressed by the conditions in the tank, let loose its chemical defenses. Not only did this kill everything in the tank (including the nudibranch that released the toxin), but it permeated my entire apartment with an extremely pungent odor. The place was unlivable until I cleaned out the tank and thoroughly aired out the apartment.
Because of these effective chemical defenses, many of these species are among the nudibranchs that are likely to be out and about in the open during the day. But even though they are toxic, their daytime exposure makes them susceptible to predation attempts by fish. Even if the fish finds the nudibranch noxious or inedible, that may still be detrimental to the nudibranch population if each potential predator has to taste one to find that out. Consequently, it appears as though what is called Mullerian mimicry has developed among some of the phyllidiids. In the typical form of the more well known Batesian mimicry, one or more (usually) innocuous species evolve to resemble poisonous or otherwise harmful species; if predators avoid the poisonous species, they may avoid the edible mimics as well. In Mullerian mimicry, more than one noxious species evolve to resemble each other. Basically, this reduces the "learning curve" of training each new predator that a particular color pattern is inedible. If several other species look like you, the predator is more likely to spend its time learning not to eat things like you by tasting someone else.
There is more information and additional photos of nudibranch mimicry in Behrens' Nudibranch Behavior.
Phyllidia
Phyllidiella
Phyllidiopsis
Reticulidia
Ceratophyllidia
