Crush some between your fingers and see if it stinks.

Until you know what you have you shouldn't rush to treat it. It might be desirable. If you plan on raising fish you will want to develop habitat.

Plus some of these (like chara) are actually algae and require an algicide while similar stuff (such as hydrilla) is really a plant and must be treated with herbicides. Identification is crucial.

4 acre pond 32 ft deep within East Texas (Livingston) timber ranch. Filled (to the top of an almost finished dam) by Hurricane Harvey 9/17. Stocked with FHM, CNBG, RES 10/17. Added 35lbs RSC 3/18. 400 N LMB fingerlings 6/18