OnMouseOver is a JavaScript event, not for displaying inline help text.
See Field Level Help for how to add your own. Snippet copied from SLDS:
<div class="slds-form-element">
<div class="slds-form-element__label">
<label class="slds-align-middle" for="form-help">Text Label</label>
</div>
<div class="slds-form-element__icon">
<a href="javascript:void(0);">
<svg class="slds-icon slds-icon--x-small slds-icon-text-default" aria-hidden="true">
<use xlink:href="/assets/icons/utility-sprite/svg/symbols.svg#info"></use>
</svg>
<span class="slds-assistive-text">Help</span>
</a>
</div>
<div class="slds-form-element__control">
<input type="text" id="form-help" class="slds-input" placeholder="Field Level Help" aria-describedby="help" />
</div>
</div>
<div id="help" class="slds-popover slds-popover--tooltip slds-nubbin--bottom-left" role="tooltip" aria-live="polite" style="position:absolute;top:15px;left:103px;margin-left:-1rem;width:20rem;">
<div class="slds-popover__body slds-text-longform">
<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Adipisci facere eligendi reiciendis obcaecati.</p>
</div>
</div>
You'll need to replace the svg with lightning:icon.