algal

Hi everyone, I have been raising 2 Horseshoe crabs for the last 5 years (1 is 3 yrs. old, 1 is 5 yrs. old) They eat well, have a big tank and are usually great but the 5 yr. old has developed Green Algal disease. His shell is turning green and splotchy, he's not eating much and moving slow. Has...