If you want to reproduce just download the latest Optifine and SEUS Renewed Shader from their websites and use them with Spiders2.0 or any other modded entity with glowing eyes. The nearest solution is waiting for the SEUS PTGI shader release or use another shader. Maybe you can add to the issue title 'glowing textures / Optifine / SEUS' or write an 'incompatiblity for SEUS Renewed' in your mod description.

I just want to write it down, if someone else has a similar problem. I think that would be a downgrade and not worth it - but I leave it to you. The only thing you could do, would be removing the emissive textures from your spider. As soon as it is modded and has emissive textures (something with glow effect - emissive lights). However, it just occurs on modded entitys. Shaders can improve or change the vector graphics in Minecraft 1.12.2, which are available for installation on any launcher and are completely free for you. Maybe you haven't done anything about it and just used the old vanilla textures. Both use glowing eyes in the night (emissive textures) and for some reason the shader can't handle/catch it and instead make the whole entity glowing. For example: The same effect is on the 'owl' in 'Exotic Birds' or on the 'wolves' from 'BetterAnimalsPlus'. For me the SEUS, but maybe on other shaders too. The shader now has a problem rendering it. It's because you use emissive lights somewhere on your spiders texture, maybe to make the eyes glowing in the night or something like that. It has to do with 'emissive textures' from Optifine in combination with the SEUS Renewed shader.

I figured out why the spiders are glowing and I thought you want to know. I have done some playing and testing this week in my modpack.

Minecraft seus shader 1.12