When I hover over the main Icon of the first ul of my nav part, the dropdown menu appears in perfect location, however, when I hover over the sub menu items, a slight indentation appears that I can't seem to find the cause for.
Hovering over the three dots "..." icon:

Hovering over the sub item of the menu:

you can clearly see the indentation when I hover over the sub menu which I colored violet upon hover over, which is exactly when the indentation will occur.
.main-navigation {
position: absolute;
right: 0px;
margin-top: -20px;
margin-right: 15px;
width: 2em;
height: 2em;
color: #D9D9D9;
border: 1px solid #D9D9D9;
/* padding: auto; */
align-content: center;
}
.firstLevelul{
padding-left: 0;
list-style: none;
justify-content:flex-end;
}
.firstLevela{
display: block;
text-align: center;
font: normal small-caps 100 20px/1.8em 'Helvetica Neue';
text-decoration: none;
}
.firstLevela:hover{
color: #D9D9D9;
width: 1.5em;
height: 1.5em;
}
.secondLevelul{
display: none;
}
.secondLevela{
display: block;
}
.secondLevelli:hover{
background-color: violet;
}
.firstLevelli:hover > .secondLevelul{
display: block;
width: 200px;
height: 200px;
background-color: white;
border: 1px solid #D9D9D9;
margin-left: -169px;
}
<nav class="main-navigation">
<ul className="firstLevelul">
<li className="firstLevelli">
<a href="#" className="firstLevela"><MoreHorizIcon className="editDropDownIcon" /></a>
<ul className="secondLevelul">
<li className="secondLevelli"><a className="secondLevela" href="#">sub menu</a></li>
<li className="secondLevelli"><a className="secondLevela" href="#">sub menu</a></li>
<li className="secondLevelli"><a className="secondLevela" href="#">sub menu</a></li>
</ul>
</li>
</ul>
</nav>
This was a little hard to work with as you didn't provide much code.
But I believe the issue is based on your height: 1.5em in the firstLevela:hover selector in CSS.
I made this incredibly rudimentary JSFiddle to demonstrate