Not surprising about the lawnmower, they actually don't really care for filamentous algae so much as the film kind of algae you get on the glass (look for the kiss marks
).
I forget what size your tank is Dan but a couple options I've found that seem to work well for me are urchins and abalones. A diadema urchin does a pretty good job but needs a LOT of space (you can get them small but it doesn't take them long to get big). Another option that has worked *incredibly* well for me is a green urchin. No idea on the species ID, looks just like a blue tuxedo but uniformly green. Anytime I've tried to google them all I find is coldwater species though, this is not a coldwater species though (sorry I realize that's not terribly helpful).