If allele frequencies for hair color are respectively 3/8 (G, green allele) and 5/8 (g, blue allele), the probablility that Mother mermaid is heterozygous for the green hair is 5/8 from Bayes theorem.

Prior probabilities of the three genotypes:

• "gg" (5/8)² = 25/64
• "Gg" 2(5/8)(3/8) = 30/64
• "GG" (3/8)² = 9/64

The probability of mermaid having genotype Gg is therefore : 1 /(1+3/5) = 5/8

Source : reference

Prior ratio (GG/Gg) Likelihood Posterior ratio

9/30 = 3/10 (daugther with green allele G) -> 2 3/10 x 2 = 3/5