Spot on pool stairs

[BeckyT]

When I opened our inground pool I noticed a black spot. It kind of looks like a burn. I figured it was some kind of stain and thought no more of it. Today I took some time to really investigate trying to clean it. I’m not sure what it is. There is a bulge under it. Almost like a screw or something coming through.

Any ideas what it is and how to repair?

 

[Texas Splash]

Is that the only spot? Is it by a return jet? Any chance something slipped in under the pool cover from winter? Maybe a rusted piece of hardware perhaps? Have you tried applying anything to it as a test yet? Rub a Vitamin C tablet on it to see if it's iron. If that doesn't work, try some dry acid in a thin sock if you have some to see if it's copper.

 

[BeckyT]

Thanks. I’ll try the vitamin c tab this evening after a quick run to Walmart. Yes. It is pretty close to a return. If something slipped in, I didn’t see it when opening the pool. It had almost no debris from last winter.
That is the only stain. It has a noticeable bump under it when you run your hand over it. I’m just hoping it’s something that we can repair.

 

[BeckyT]

My teenage daughter says it was there last year. I don’t remember seeing it before.

 

[Texas Splash]

My teenage daughter says it was there last year. I don’t remember seeing it before.

My kids don't look-up away from their phones long enough to see anything. Okay, we'll wait to see what the Vitamin C test reveals if anything.

 

[BeckyT]

Is that the only spot? Is it by a return jet? Any chance something slipped in under the pool cover from winter? Maybe a rusted piece of hardware perhaps? Have you tried applying anything to it as a test yet? Rub a Vitamin C tablet on it to see if it's iron. If that doesn't work, try some dry acid in a thin sock if you have some to see if it's copper.

I thought I replied already. So this may be a duplicate. Yes. That is the only stain and it is by a return jet. I will try the vitamin c tab this evening. I’ll have to run to Walmart to get one.