You should be able to use description or description-id if you want to use an i18n property instead of putting the description in the form config.
You can also use help and help-id, but those don't make tooltips. Using one of those creates a question mark next to the property that hides/shows help text below the field control.
It looks like you are using a custom control. I'm not sure what you've done there, but I recently did a custom multi select control based on the out-of-the-box selectmany.ftl and I believe I had to tweak it to have a tooltip per entry by putting a title attribute on every option element.
Something like:
<#list field.control.params.options?split(optionSeparator) as nameValue>
<#if nameValue?index_of(labelSeparator) == -1>
<option value="${nameValue?html}"<#if isSelected(nameValue)> selected="selected"</#if>>${nameValue?html}</option>
<#else>
<#assign choice=nameValue?split(labelSeparator)>
<#assign choiceTitle="{form.field.description." + choice[0] + "}">
<option value="${choice[0]?html}" title="${msgValue(choiceTitle)?html}" <#if isSelected(choice[0])> selected="selected"</#if>>${msgValue(choice[1])?html}</option>
</#if>
</#list>