The claims about how much B12 ulva contains don't really match each other... claims that ulca contains 8.6 micrograms B12 / 1g dried ulva, another source, which sells dry ulva for cattle feed suggests 7g B12/100g ulva, and then there are these sources: :

In Ulva sp vitamin A in 100g dry matter was 960 retinal U and B complex (B1, B2, B3, B12 and Folic acid) was (0.06, 0.03, 8.0, 6.3 and 11.8 mg, respectively) :
Ulva lactuca can provide this vitamin in excess of the recommended dietary allowances for Ireland of 1.4 mcg/day with 5 mcg in 8g of dry foodstuff. This may provide an alternate source of vitamin B12 for vegetarians or vegans.