BE-65 pilot reported that the aircraft would not maintain altitude and suspected they were in clear icing conditions. Pilot exited icing conditions and continued flight with no further issues.

Narrative

I was flying at night in a Aircraft X. I had been in IMC and snow most of the flight but I started to see clear ice build on the wings. I had never experienced clear ice before. As it was night I was watching this via the ice light. One thing I'm sure of after the event is that it was clear ice. The severity and the cause I'm not sure of. At the time it was precipitating and it wasn't snow; I thought it might be freezing rain. My airspeed was not bleeding; I was pretty steady between 165 and 160 knots indicated. As I'm watching the clear ice and trying to shed it via the boots I noticed that my altitude was decreasing. I noticed it at about 8;800 feet and contacted Denver Center as soon as I was able to ask for a lower altitude. I told the controller that I was getting moderate clear ice and was unable to hold my altitude. He cleared me to 6000 feet and asked me to tell him when the icing ended. I broke out at 7800 and it was below freezing at 6000 feet. I had no more issues afterward. When I landed I called Person X to describe the event to him and ask if I might have experience freezing rain. Person X told me that based on my airspeed that it was likely an autopilot servo issue I did monitor the autopilot and tested it when I landed at the end of the night with no issues. On the way back; I experience rime ice; which I've had experience with; and was able to determine that I had experience clear ice and that I wasn't having any autopilot issues as a result. Airspeed at 10000 on the way home was between 150 and 152 knots indicated. There was precipitation on the way back but it was solely snow. As far as the altitude discretion - I was on autopilot and was focused on the ice. I did notice the altitude decreasing and did contact Center before I was in violation. As a single pilot and my first experience with clear ice; I'm not disappointed with my reaction; but I was more focused on the ice than the altimeter at the time. As far as the why; I think it was a servo issue; but I think the issue was due to the ice. The autopilot should have continued trimming the plane out to maintain altitude but it didn't. I verified that the autopilot was engaged and checked altimeter settings before making any contact with ATC. I did not try to climb; I knew that the temperature was above freezing below and I knew that I'd break out because I heard a Aircraft Y doing a practice approach below so I asked if I could get lower which I was granted.