Are federal medical professionals able to be sued for malpractice?

If the medical professional is negligent in the performance of his/her professional duties within the course and scope of his/her duties as a government physician, nurse or other professional, he or she is protected by a concept called "sovereign immunity". It affords limited protection for negligence to the extent that the law requires several steps to be taken before a lawsuit can be filed. Most of these have to do with pre-suit notice and sometimes, a more limited time within which to file suit (than would be the case were a private physician to be subject to suit). Additionally, applicable law may limit the damages that can be awarded, and the suit would have to be filed in federal (rather than state) court.
If you are referring to medical professionals, such as physicians or government hospitals, who are employed or run by the Federal Government, yes, they are (like other physicians). The main difference is that in suing any government entity or person working on behalf of the government for actions taken (or that should have been taken, but weren't), a concept called "soverign immunity" comes in. This has long and deep historical roots.
In modern times, the concept has been modified by statute such that the Federal (and State) governments have granted permission, in must situations, to allow themselves to be sued and to answer in money damages in much the same way as a similiarily situated person or entity would. However, often, the statutes that grant permission to sue limit the amount of money that can be collected; to collect more (such as if a jury verdict exceeded the statutory limitation from the government, a Claims Bill would have to be filed and passed in the appropriate jurisdiction. The Claims Bill is initiated by a House member or Senator (Federal or State, as appropriate), who takes up the cause of obtaining compensation for the person(s) who were awarded the judgment.

In most all situations where a government entity or operative is to be sued, a pre-suit notice is required. The statutes governing this topic prescribe very specific time-lines which, if not strictly followed, may preclude suit. Often, once the conditions precedent to suit have been complied with, suit must be filed more quickly than otherwise prescribed by a governing statute of limitations were a non-government entity or operative involved.