You need a California attorney to answer this question, but here's a general point about business names: In many states, there is some flexibility to how a business is named in a contract and within limits it can be considered sufficient. This is so, in part, because so many businesses do not use their own full, legal name at all times when doing business. Your California attorney should ask you the question: Who prepared the signature blocks on the contract form? If it was the LLC that prepared it and choose to put some variant on its own name on the form, then it would be a poor result to have a court let them off the hook by their own sloppiness. A second question for your lawyer to ask is whether the company ever calls itself, simply, "Hello," or has any literature that just says, "Hello, LLC." Many businesses have a place name as part of their name to distinguish them from others but then, it practice, answer the phone and put things in writing without using the full, legal name.
Mr. Richello gives some good analysis.
One cannot predict what any court will do. If they have filed a demurrer, you will need to file an opposition at your earliest occasion and make all your best arguments why the demurrer should fail.
You will get much better analysis from an attorney who interviews you. Sometimes an answer leads to a question one would not have thought to ask.
The above is general legal and business analysis. It is not "legal advice" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
No one on AVVO could predict what a court will do, especially without having reviewed all the relevant documents. In any event, you need to respond to the demurrer. You can do that by opposing it, or if you have not done so yet, by amending your complaint to correct any defects.
At the very least, you should talk to an attorney with experience in construction cases to make sure that you are not overlooking something or giving up some of your rights.