I found that the little (1/8 inch) snails would eat holes in my ulva. FWIW if you want the ulva to grow, I've had better luck in a separate tank that sits on the window sill without a heater. In nature it grows on the shores of Ireland, etc = cooler water. I just add some f2 fertilizer from Florida Aqua Farms. Once or twice a week, I break some off and throw into the display tank for the tang.
Vickie