Cut by Flower Crab

I have a fairly big flower crab in my aquarium.  It is usually well camouflage during the day.  Algae grows fast on the side of the aquarium.  So I have to clean the glass regularly in order to see my fishes.  I was cleaning as usual yesterday.  B1 was talking to me and suddenly a sharp pain and I shake it off.  And I continue to clean but the pain persist.  I thought it is quick pinch.  When I looked at my fore finger, there is a red line which is about 2.5cm long and 3 mm thick.  So no problem, I thought.  But when I squeeze it, blood start oozing out quickly.  The quick pinch cuts right into my skin!  Now my finger is throbbing with pain.  It is hurts but still bearable.