According to Minecraft Wiki, mobs cannot spawn on farmland, but safe overlay is still rendering yellow/red crosses over farmland blocks.

Here's the screenshot.(Ground level at y = 4) No mobs are spawning on farmland.

The screenshot is taken in Mineraft 1.8 with Zyin's hud 1.4.6, but the problem exists for minecraft 1.8.8 and Zyin's hud 1.4.8 too.

Thank you for your time!