If an employer sent a health insurance company monies, the insurance company deposited that money in its account, is the interest earned from those monies considered a plan asset?

It is possible, but probably not. The way most A&H plans are constructed today, that money on deposit is an asset of the employer and not a plan asset so the interest on the money would also not be a plan asset. If the money was strictly from employee contributions or a combination of employer and employee contributions then it could be a plan asset.