Add custom tick on and a label y-axis with custom colour using ggboxplot()

I have a boxplot that I've created using ggboxplot. On it I have a horizontal line at the mark y=50. I want to be able to add a label at y=50, x=0 saying "50" in dark green to match the line. Currently the label overlaps the axis line and the boxplots have shifted to the right. Any suggestions?

Problem

Current Code

symnum.args <- list(cutpoints = c(0, 0.0001, 0.001, 0.01, 0.05, 1), symbols = c("", "", "", "", "ns"))

my_comparisons<- list(c('BVR3', 'BVR4'), c('BVR6', 'BVR1'), c('BVR6', 'BVR2'), c('BVR6', 'BVR4'), c('BVR6', 'BVR5'))
plot <- ggboxplot(TPdat, x = "loc", y = "TP", 
          ylab = "Median Annual Total Phosphorus (\U00B5g/L)", xlab = "Sampling Site", 
          outlier.shape=NA, ylim=c(0, 925)) +
          scale_x_discrete(labels=c('1','2','3','4','5','6')) +
  theme(axis.text=element_text(size=16, family='Calibri'),
        axis.title=element_text(size=16, family='Calibri')) +
  geom_hline(yintercept=50, col='dark green', linetype=2) +
  coord_cartesian(clip='off', ylim=c(0, NA)) +
  annotate("text", x=0, y=50, label="50", colour='dark green', size=5) +
  stat_compare_means(comparisons=my_comparisons, label='p.signif', 
                     symnum.args = symnum.args, 
                     tip.length = c(0.01,0.01,0.01,0.01,0.01), 
                     label.y=c(800, 825, 850, 875, 900)) +
  stat_compare_means(label.y=930, family='Calibri', size=5)

head of dataframe

structure(list(loc = structure(c(1L, 1L, 1L, 1L, 1L, 1L), levels = c("BVR1", 
"BVR2", "BVR3", "BVR4", "BVR5", "BVR6"), class = "factor"), TP = c(76L, 
71L, 86L, 48L, 58L, 73L)), row.names = c(NA, 6L), class = "data.frame")

Based on alistaire's answer suggested here Change color of specific tick in ggplot2 we could do:

library(ggplot2)

    ggplot(iris, aes(x=Species, y=Sepal.Length)) + 
      geom_boxplot() +
      scale_y_continuous(breaks=c(5,6,7,7.5,8), 
                         labels=expression("5", "6", "7", "7.5", "8")) +
      geom_hline(yintercept=7.5, col='dark green', linetype=2) +
      theme(axis.text.y = element_text(colour = c('black', 'black','black', 'dark green', 'black')))

I used the iris dataset to ilustrate how to get one specifick tickmark colored green. Hope it helps

Boxplot with one tickmark green:

This method works with ggpubr, too:

ggboxplot(iris, x = "Species", y = "Sepal.Length") +
  scale_y_continuous(breaks=c(5,6,7,7.5,8), 
                     labels=expression("5", "6", "7", "7.5", "8")) +
  geom_hline(yintercept=7.5, col='dark green', linetype=2) +
  theme(axis.text.y = element_text(colour = c('black', 'black','black', 'dark green', 'black')))