Given the size of the coin and the reverse legend that is visible, I think it's one of these: Helena, Augusta AD 324-328/330. Roman billon reduced centenionalis, 1.24 g, 13.4 mm. Trier, AD 337-340. Obv: FL IVL HELENAE AVG diademed and draped bust, right. Rev: PAX PVBLICA, Pax standing left, holding olive branch and transverse scepter; in exergue TR S. Refs: RIC viii, p. 144, 78; LRBC I 119; Cohen 4; RCV 17492.
Looks like it might be a commemorative coin of Helena (A.D. 324 - 330?), minted by her grandsons (Constantine II, Constantius II and Constans) around 337-340. If so, the reverse would be PAX PV-BLICA, Pax standing left, holding branch downward in right hand and traverse scepter in left, like on the coin in this photo from Wildwinds: The coin looks like it would clean up pretty nicely. Try soaking it in distilled water (not tap water) and scrubbing it with a toothbrush and maybe a little dishwashing liquid.