I had a bm that was brown and medium sized. it was thickly coated in what looked like brown algae with sand-like sediment falling off of it. it was an easy, soft bm. 3/10 pain during. is this "algae" of any concern?

Since you mentioned some pain during the BM a possibility is the presence of an anal fissure and the substance on it could be mucus, some of which could have solidified. If anal discomfort persists have a rectal examination by your physician, a proctologist or colorectal surgeon for evaluation and advice. In the meantime keep BM's on the soft side, apply warm water compresses to the anus as needed