Here is the lobster slider description from AllEars:
New England Style Lobster Sliders - served on a butter toasted rolls with a choice of Vegetable Orzo Salad or fries $16.99
There were some photos floating around of the old lobster roll at CG, before they switched to the sliders, but it looks like some of the photos were not actually from CG.
Do you know know what a lobster roll is in general? There are 2 traditional lobster rolls. The main ingredient is obviously lobster. Some places will just have lobster, in butter, served on a roll. Other places use lobster with a little mayo (that is the key, some places put way too much mayo, but in New England, you want to taste the lobster, not the mayo, so less is more) and maybe a little celery. The traditional lobster rolls are served on a buttered and grilled New England style hot dog roll.
I'm guessing that the sliders are pretty much the same (lobster and mayo), but served on a slider roll (a small square or round roll). If I find a picture, I'll post it.
I did see some reviews on the Disney Food Blog for Captain's Grill too. There were some photos, but not any of the sliders.