I think you need to wait about 1-2 weeks and post new pictures. All hydrangeas look very similar to each other when the blooms are in the broccoli stage. Once the blooms have opened some, post a close-up or two of them. By the way, is the mophead supposed to have white blooms? I saw one bloom on the far right-hand side of the picture that seems partly opened and white but it is not clear and I wanted to confirm with you. If so, consider these well-known white rebloomers: Blushing Bride, Wedding Gown, White King, White Out. A f.y.i.: Mme. Emile Mouillere, also a white mophead, sometimes reblooms during the Fall but is not in the same "reblooming class" as the others and is not formally considered a rebloomer. Here in Texas, it has done so sparingly for me.

Nice going! You got a $30+ shrub for a bargain. That leaf shape color and overall shrub size points to Mme Mouillere...

It's remotely possible it is an ES Blushing Bride but if it's older than 7-8 years it's likely a MEM. Mine are just babies but are outgrowing their pots and will soon go in the ground. Strong shoots have come from the base and have buds to boot! A superior shrub. Sun tolerant.

Doesn't look very much like my Mme. Emile Mouillere!! I agree with Luis that waiting until the flowers are in full bloom and then reposting a pic may provide better results. MEM should have a very distinctly colored but small 'eye' in the center of the sterile flowers - blue if planted in acidic soil and pink if more neutral to alkaline.

I would wonder how likely it is to be one of the reblooming/old and new wood bloomers......all of these are relatively new, patented plants and are illegal to propagate unless licensed.....

I am starting to think that what I have is an Endless Summer Hydrangea. The blooms are now turning pink and by the bud pattern and left shape and size. I have posted a link to some very recent pictures.
Thank you to everyone that responded.

Here is a link that might be useful: Endless Summer???

Well here are some up to date pictures of this mysterious Hydrangea. The flowers never really got larger than my fist and pretty much stayed true to the white color. It did show some shades of very light pink but never turned. Upon further investigation, this plant blooms on old wood only. We have irrigation that spray's on the plant and not on the root so I believe this to be the reason the flowers have not remained their beautiful color and form. This will have to be changed to a different type spray head to avoid this next year. Any input would greatly be appreciated!

Here is a link that might be useful: Mystery Hydrangea

looks like my MEM......