u/Free-Doubt68

So my gerbil for the last couple days has been slow going and has recently perked up a bit. They’re getting a little old 1yr 4mth but he has been moving around more since the past two days. Today I noticed that his eye had whatever this is around it, I had noticed that one of his eyes was a bit more shut but thought nothing of it. Is it anything serious if treatable advice on how would be greatly appreciated! (His brother is also perfectly fine)