Must the clerk accept a proper filing?

According to statute, generally yes:


§ 2-201. In general
(a) Enumeration. -- The clerk of a court shall:
(1) Have custody of the books, records, and papers of his office;

(2) Make proper legible entries of all proceedings of the court and keep them in well-bound books or other permanent form;

(3) When requested in writing to do so, record any paper filed with his office and required by law to be recorded in the appropriate place, whether or not the title to land is involved;

(4) Unless prohibited by law or order of court, provide copies of records or papers in his custody to a person requesting a copy, under the seal of the court if required;

(5) Issue all writs which may legally be issued from the court;

(6) Deliver a full statement of the costs of a suit to a party requesting a copy;

(7) Receive all books, documents, public letters, and packages sent to him pursuant to law, and carefully dispose of them as the law requires;

(8) Administer an oath;

(9) Replace worn books and records with new ones;

(10) In conjunction with the Motor Vehicle Administrator, establish uniform procedures for reporting both traffic cases and criminal cases involving a motor vehicle in the circuit court to the Motor Vehicle Administration; and

(11) Perform any other duty required by law or rule.

(b) No duty to record until costs paid. -- Unless otherwise provided by law, a clerk is not required to record any paper filed with him or to provide any person with a copy of a paper until the applicable charge has been paid.
MD Code Cts. & Jud. Proc. 2-201 In general (Maryland Code (2019 Edition))

Act of filing papers with court is as ministerial and inflexibly mandatory as any of clerk's responsibilities. Maccray versus Maryland 456  F2d 1 (Fourth Circuit 1972)

There are some restrictions, the item must be in proper form, generally there must be a certificate of service, if these need to be paid, etc. but in general the court must accept documents for filing.

What does "shall" mean?

In Maryland there is some debate as to the term shall. Sometimes I like to equate shall with must. Excerpts from this opinion seem to support that position:

Lovero v. Joelma Da Silva., 200 Md.App. 433, 28 A.3d 43 (Md. App., 2011

It is important to note that in each iteration of the rule the clerk is directed not to “file” any paper or pleading requiring service that does not contain the appropriate proof of service. See Rule 1–323 (“The clerk shall not accept for filing....”); Rule 306 a.2. (“The clerk shall not accept or file ....”); and Rule 1(a)(2) (a paper “shall not be received and filed by the clerk ...”). The use of the word “shall” in a rule means that the conduct proscribed is mandatory. See Rule 1–201(a); 5 Owens v. Prince George's Cnty. Dep't of Soc. Servs., 182 Md.App. 31, 43–44, 957 A.2d 191 (adopting the view that the use of the word “shall” with respect to commands found in the Maryland Rules is mandatory), cert. denied, 406 Md. 746, 962 A.2d 372 (2008); Tranen v. Aziz, 59 Md.App. 528, 534, 476 A.2d 1170 (1984) (explaining that the term “shall” in a rule “is presumed mandatory ..., denoting an imperative obligation inconsistent with the exercise of discretion” (citation and quotations omitted)), aff'd, 304 Md. 605, 500 A.2d 636 (1985). Therefore, it is clear that in adopting Rule 1–323, and its predecessors, the Court of Appeals intended that a

        [28 A.3d 50]

pleading or paper requiring service that did not contain the appropriate proof of service was not to become a part of any court [200 Md.App. 446] proceeding by being “filed” in the court file of such proceeding.
Lovero v. Joelma Da Silva., 200 Md.App. 433, 28 A.3d 43 (Md. App., 2011)