I don't get it. Why does hargreave say that "if we designed the suit correctly, it should work". But when he entered that giant spear, his suit started to go odd. Like black spots or something. Same when he was punching the hive, he started having some weird black spots. What are they, what do they mean? What does Hargreave mean?

The spore was eating through the suit but the suit could repair itself at a faster rate. The suits job was to act as a bridge between human and ceph technology allowing the suit to sabotage their bio weapons.