At the risk of making one of those wishy-washy "in my experience" posts, every Dolabella sea hare I've kept has not been content with a single type of food. They will go through phases where they've had enough of one thing and want something else when well-fed. My current Dolabella is going through a no-hair-algae time and only shows interest in algae wafers and nori. A couple weeks ago it was hair algae all the way and little interest in nori.
There is also an article I've read recently, "Selectivity and Growth of the Generalist Herbivore Dolabella auricularia Feeding upon Complementary Resources." by Pennings et al. (in Ecology, 1993) that addresses some interesting issues of diet. Some relevant info from the abstract in case the pdf isn't easily accessible:
"(...)The sea hare Dolabella auricularia (Gastropoda: Opisthobranchia: Anaspidea) grew far better upon an ad libitum mixture of four species of algae than upon any of the algal species alone, suggesting that algae were complementary resources. (...) Preference for a species of algae was always higher when it was not the maintenance diet compared to when it was. (...)"
So, perhaps your sea hare is just exploiting a resource while it lasts (the algae on the glass) and will go back to the hair algae later.
