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.