Air carrier Captain reported receiving a low altitude alert from ATC on the Stadium Visual approach to EWR.

Narrative

We were flying the Stadium Visual Runway 29 approach with a speed restriction of 170 kts. to SLIMR. The approach was well-briefed and the automation was set up in LNAV VNAV Path; autopilot and auto throttle engaged with touchdown zone elevation set in the MCP altitude window immediately after GIMEE. Additionally; we had inserted COPKO on the legs page after SLIMR and before CHUMR. We'd planned on being in final landing configuration by SLIMR and we were able to have the gear down and the flaps to 15 but we held off the final flaps momentarily approaching SLIMR due to the ATC speed restriction and the lower flap limiting speeds of [this] aircraft. At this point; I became aware that the approach happens more quickly than I expected and that we had a manageable amount of excess energy. Passing SLIMR; we reduced airspeed but two unusual things happened. First; the Tower Controller advised us to slow to final approach airspeed and said 'Low Altitude Alert'. I'm still unsure if the low altitude alert was for us because it looked like we were high if anything. The other thing was our vertical guidance went away and the autopilot reverted to [a degraded mode]. I instructed my First Officer to turn off the autopilot. He did; and did a great job of hand flying from there. I got the flaps to final and the checklist complete. We descended to and captured a normal visual glide path by approximately 500 ft. AGL. The rest of the landing was uneventful.