Airstone in HOB

[Jeff]

I took Corey's advice from one of his tips videos. I added a never clog air stone in the back of my AquaClear HOB. 
 

I haven't noticed until now, but it has created a black mark on the wall behind it. This hasn't come about until I added the airstone. 
 

Why would an airstone do this, and any tips on stopping this from happening after I paint over it? 
 

Note: I keep the lid loosely on the HOB. The pic of the HOB is just for reference  

Edited December 28, 2022 by Jeff

[egruttum]

As the bubbles rise to the surface and pop they make a little bit of spray.  I'll bet the spray is coming out of your lid and hitting the wall.  You probably never notice it because the spray is so fine that it evaporates pretty quickly so nothing stays wet.  But any small particles in the water that are either suspended (TSS) or soluble are then deposited on the wall when the spray dries.  Taking the air stone out should solve the issue.  Or maybe painting the wall with a paint that's easily wipe-able would allow you to clean it on a regular basis before it becomes a black mark.  Or get a tighter fitting lid.