Measure of liquid being seaweedy, litre one’s about to drink (8,6)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

I’M =I am=one is=”one’s” + ON =regarding=”about”, both around/”to drink” all of: PERI =a supernatural “being” in Persian folklore; plus ALGAL =like algae=”seaweedy”; plus L (litre)