It doesn't seem to me S-05 should respond any differently than just about any other American Ale Yeast, in general, dry or liquid, and while I personally do not have experience with 05, I do use WLP001 quite often, I believe they are the same strain, so it is from that I will draw my conclusions.
Lemon Verbana is a leaf that is not that far away from hops, similar in that it contains essential oils that, with a 5 minute boil, will evaporate somewhat but will still add a lemony note to your brew. One of the essential oils happens to be geraniol which is converted to citronellol during fermentation, so I will assume you're going to have some nice citrus flavors in this one! But I digress; this still should not cause your yeast to act differently.
Ale yeasts are top-fermenters and as such, they tend to hang out on the top of the wort during the active phases of fermentation, but there will still be plenty of cells that drop to the bottom and some stay there and when they begin to accumulate, a "cake" is formed. It would be difficult for such a cake to form suspended at the top of the liquid - too heavy - it would drop to the bottom.
Are you sure what you're seeing on the top is not just a nice healthy krausen - a thick, active foam layer, often white with brown spots? How long has it been fermenting? What temperature are you maintaining?